Extrapulmonary tissue responses in cynomolgus macaques (Macaca fascicularis) infected with highly pathogenic avian influenza A (H5N1) virus.

The mechanisms responsible for virulence of influenza viruses in humans remain poorly understood. A prevailing hypothesis is that the highly pathogenic virus isolates cause a severe cytokinemia precipitating acute respiratory distress syndrome and multiple organ dysfunction syndrome. Cynomolgus macaques (Macaca fascicularis) infected with a human highly pathogenic avian influenza (HPAI) H5N1 virus isolate (A/Vietnam/1203/2004) or reassortants of human influenza virus A/Texas/36/91 (H1N1) containing genes from the 1918 pandemic influenza A (H1N1) virus developed severe pneumonia within 24 h postinfection. However, virus spread beyond the lungs was only detected in the H5N1 group, and signs of extrapulmonary tissue reactions, including microglia activation and sustained up-regulation of inflammatory markers, most notably hypoxia inducible factor-1alpha (HIF-1alpha), were largely limited to this group. Extrapulmonary pathology may thus contribute to the morbidities induced by H5N1 viruses.

Metabolism of thalidomide in human microsomes, cloned human cytochrome P-450 isozymes, and Hansen's disease patients.

Previous in vitro studies in rat microsomal preparations suggested that thalidomide is metabolized by the cytochrome P450 system (CYP). In this study, we examined the extent of thalidomide metabolism by preparations of pooled human microsomes, microsomes containing cloned human CYP isozymes (CYPIA2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4), and Hansen's disease patients. Results indicated that thalidomide was a poor substrate for CYP isozymes. Alteration of incubation buffer, pH, incubation time, and microsome and thalidomide concentrations did not increase the production of any metabolites. Thalidomide also did not inhibit metabolism of CYP-specific substrates and therefore any interactions with other drugs that are metabolized by the same enzyme system are unlikely. Hansen's patients were given a single oral dose of thalidomide (400 mg), and their blood and urine were collected at time points up to 72 hours, processed, and analyzed by tandem mass spectrometry. Although thalidomide was present in the plasma and urine, no metabolites were found in the plasma and very low amounts of the 5-OH thalidomide metabolite were present in the urine. These results suggest that thalidomide does not undergo significant metabolism by human CYP and that clinically important interactions between thalidomide and drugs that are also metabolized by this enzyme system are unlikely. The major route of thalidomide breakdown in humans and animals is through spontaneous hydrolysis with subsequent elimination in the urine.

Physiologically based modeling of 2-butoxyethanol disposition in rats following different routes of exposure.

2-Butoxyethanol (BE) is widely used as a solvent in coatings and other consumer products and has shown hematotoxicity in laboratory animals. To provide a physiological basis for extrapolating toxicokinetic data observed in rats to humans, a blood flow rate-limited, physiologically based pharmacokinetic model was developed to describe the distribution and metabolism of BE in rats following drinking water, dermal, and inhalation exposures. The major urinary metabolite, butoxyacetic acid, represented 45 to 60% of the absorbed dose in all three routes of exposure. Other identified urinary metabolites in our studies included ethylene glycol and BE-glucuronide. A model formulation of the possible metabolic pathways based on the experimental data was proposed. The amounts of individual urinary metabolites were used to develop the model. Metabolic constants were estimated by fitting the data within the constraints of in vitro measurements. The model explained the change of profiles of urinary metabolites in different exposure routes by taking into account the differences in absorption rate and by incorporating a minor pathway for metabolism by skin. Sensitivity analysis showed that metabolic constants and blood flow rate to liver had a relatively larger influence on the production of urinary metabolites than the organ volume or the partition coefficient for BE.

Disposition of inhaled isobutene in F344/N rats.

Isobutene (2-methylpropene) (CAS No. 115-11-7) is a gas widely used in the chemical manufacturing industry. As an aid to planning long-term toxicity studies, research was conducted to determine the effect of exposure concentrations on the absorption and metabolism of isobutene in F344/N rats. Male F344/N rats (11-15 weeks of age) were exposed for 2 hr to 0, 40, 400, or 4000 ppm isobutene, and a time-course evaluation of blood levels of isobutene was performed using headspace analysis methods. Blood levels of isobutene were linearly related to exposure concentrations between 40 and 400 ppm but increased in a supralinear fashion at the highest concentration, suggesting that the capacity of the rats to metabolize isobutene had been exceeded. Total uptake, excretion patterns, and metabolic conversions were studied in rats exposed for up to 6 hr to 0, 2, 40, 400, or 4000 ppm [14C]isobutene. Absorption of the inhaled isobutene was approximately 8% up to 40 ppm isobutene, but decreased at the higher concentrations. The amount of isobutene metabolized per ppm.hr of exposure was also linear up to 40 ppm but decreased at higher concentrations. Over 90% of the absorbed isobutene was metabolized at exposure concentrations up to 400 ppm, but the exposure to approximately 4000 ppm isobutene resulted in approximately 20% of the absorbed dose exhaled as the unmetabolized isobutene. Two urinary metabolites were identified as isobutenediol and 2-hydroxyisobutyric acid. Two other urinary metabolites were tentatively identified as sulfate conjugates of isobutenediol. Based on these studies, linear dose-response relationships would be expected in chronic toxicity studies for exposures up to 40 ppm isobutene. Additional studies would be required to determine if repeated exposures would induce higher metabolic capacities in the exposed rats.

Disposition of polycyclic aromatic hydrocarbons in the respiratory tract of the beagle dog. II. The conducting airways.

Physiological models have predicted that the lipophilicity of solutes such as polycyclic aromatic hydrocarbons (PAHs) will delay clearance from the respiratory tract. This clearance consists of a delayed penetration of the mucous lining layer (MLL), allowing mucociliary clearance, followed by a slow penetration of PAHs through walls of the conducting airways. To test this prediction, mucociliary clearance and retention in the mucosa of PAHs deposited in the conducting airways of the Beagle dog were measured. Mucociliary clearance of particles and dissolved PAHs was measured by instilling onto the MLL in a main stem bronchus or the distal trachea small volume of saline containing either dissolved benzo(a)pyrene (BaP) or phenanthrene (Phe), or a suspension of particulate solvent green (SG) or macroaggregated albumin (MAA). Sequential lavage of the mucous-retained materials followed the instillations. Retention of BaP in the airway walls of the bronchial tree was studied by instilling the hydrocarbon in an ethanol/saline solution at precise locations of the upper bronchial tree, and measuring the concentration of BaP and its major metabolites in the tissues. Results indicated that mucociliary clearance of SG and MAA particles in the trachea of the Beagle dog occurred at average rates of 27-30 mm/min. Of the two solutes, only the highly lipophilic BaP was sufficiently retained within the MLL to be transported with the mucociliary escalator. In addition, a fraction of the lipophilic materials cleared at a very rapid rate, in excess of 90 mm/min. This may indicate that one monolayer of pulmonary surfactant at the air interface is spreading out of the lungs on top of the MLL ata faster rate than mucociliary clearance. However, despite the protective properties of the MLL, fractions of BaP penetrating to the bronchial epithelium had a clearance half-time in the range of 1.4 hr, a period during which considerable metabolism of the PAH occurred. This long retention indicates a diffusion-limited uptake of BaP by the airways, and underscores the potential for local toxicity of highly lipophilic toxicants in the bronchial epithelium.

Species differences in the metabolism of 1,3-butadiene in vivo.

The metabolism of 1,3-butadiene in vivo was studied in mice, rats and primates. The percentage of inhaled butadiene that is absorbed by exposed animals depends in large part on its rate of metabolism. Uptake of inhaled butadiene was 20% in B6C3F1 mice but only 4% in Sprague-Dawley rats and 3% in cynomolgus monkeys exposed to low levels (10 ppm 14C-butadiene or less). The routes of excretion of the carbon-14 retained after completion of the exposures were similar in rats and mice, one-half of the material being excreted in urine, 5-10% in faeces, 5-10% exhaled as carbon dioxide, 15-20% exhaled as volatile metabolites and 10-20% retained in the body. Monkeys, however, appeared to metabolize the retained 14C-butadiene more completely, as one-half of the internal dose of butadiene was exhaled as 14C-carbon dioxide. With equivalent exposures, blood metabolite levels were much higher in mice than in monkeys. In mice, the only species examined thus far, bone marrow was found to have much higher levels of butadiene monoepoxide per gram of tissue than did the blood, suggesting formation of the butadiene monoepoxide in the marrow in situ. The major urinary metabolites were two mercapturic acids (called M-I and M-II), formed from the glutathione conjugates of either butadiene monoepoxide (M-II) or the butenediol hydrolysis product of butadiene monoepoxide (M-I). Mice excrete three times as much M-II as M-I, corroborating the finding in vitro that mice are more efficient at forming the glutathione conjugate of butadiene monoepoxide than in hydrolysing it to the butenediol.(ABSTRACT TRUNCATED AT 250 WORDS)

Species differences in urinary butadiene metabolites; identification of 1,2-dihydroxy-4-(N-acetylcysteinyl)butane, a novel metabolite of butadiene.

1,3-Butadiene (BD) is used in the manufacture of styrene-BD and polybutadiene rubber. Differences seen in chronic toxicity studies in the susceptibility of B6C3F1 mice and Sprague-Dawley rats to BD raise the question of how to use the rodent toxicology data to predict the health risk of BD in humans. The purpose of this study was to determine if there are species differences in the metabolism of BD to urinary metabolites that might help to explain the differences in the toxicity of BD. The major urinary metabolites of BD in F344/N rats, Sprague-Dawley rats, B6C3F1 mice, Syrian hamsters, and cynomolgus monkeys were identified as 1,2-dihydroxy-4-(N-acetylcysteinyl)-butane (I) and the N-acetylcysteine conjugate of BD monoxide [1-hydroxy-2-(N-acetylcysteinyl)-3-butene] (II). These mercapturic acids are formed by addition of glutathione at either the double bond (I) or the epoxide (II) respectively. When exposed to approximately 8000 p.p.m. of BD for 2 h, the mice excreted 3-4 times as much metabolite II as I, the hamster and the rats produced approximately 1.5 times as much metabolite II as I, while the monkeys produced primarily metabolite I. The ratio of formation of metabolite I to the total formation of the two mercapturic acids correlated well with the known hepatic epoxide hydrolase activity in the different species. These data suggest that (i) the availability of the monoepoxide for conjugation with glutathione is highest in the mouse, followed by the hamster and the rat, and is lowest in the monkey; and (ii) the epoxide availability is inversely related to the hepatic activity of epoxide hydrolase, the enzyme that removes the epoxide by hydrolysis. The ratio of the two mercapturic acids in human urine following BD exposure may indicate the pathways of BD metabolism in humans and may aid in the determination of the most appropriate animal model for BD toxicity.

Effect of dose on the disposition of methoxyethanol, ethoxyethanol, and butoxyethanol administered dermally to male F344/N rats.

The glycol ethers methoxyethanol (ME), ethoxyethanol (EE), and butoxyethanol (BE) are widely used in industrial and household products. Rodent studies indicate the ME and EE are potentially toxic compounds causing teratogenic, fetotoxic, hematotoxic, and testicular effects. Exposure of rodents to high concentrations of BE resulted in anemia due to hemolysis of blood cells, leukopenia, hemoglobinuria, and liver and kidney damage. The purpose of this study was to determine the uptake, metabolism, and excretion of dermally administered glycol ethers as a function of the externally applied dose. Three different amounts of the 14C-labeled glycol ethers (450-4000 mumole/kg) were applied to same-sized areas on the clipped backs of F344/N rats, and nonoccluded percutaneous absorption was measured. The rates of excretion of the 14C-labeled parent compound and metabolites by different routes were measured, as well as the amount of 14C remaining in the carcass. Within the dose range studied, the absorption and metabolism of these three glycol ethers by F344/N rats was linearly related to the dermally applied dose. The absorption of all three glycol ethers was approximately 20-25%, regardless of the chain length of the alkyl group or the dose administered. The majority of the absorbed dose was excreted in the urine. Feces and exhaled CO2 represented minor routes of excretion. The alkoxyacetic acid was a major metabolite for all three glycol ethers. The formation of small amounts of ethylene glycol indicated cleavage of the ether bond.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of exposure concentration on the disposition of inhaled butoxyethanol by F344 rats.

The glycol ethers are a class of solvents widely used due to their range of vapor pressures and miscibility in aqueous and organic media. Butoxyethanol (BE) causes anemia and lowered hematocrits in rats due to direct hemolysis of red blood cells. Exposure to BE is most likely to occur by dermal contact or by inhalation. In this paper, we report the uptake, metabolism, and excretion of BE following 6-hr exposure at different inhaled concentrations. The uptake and metabolism of BE were essentially linear up to 438 ppm. The majority of the inhaled butoxy-[14C]ethanol was eliminated in the urine with butoxyacetic acid (BAA) being the major urinary metabolite, accompanied by lesser amounts of ethylene glycol and BE glucuronide. A small proportion (5-8%) of the retained BE was exhaled as 14CO2. Most (greater than 80%) of the [14C]BE-derived material in blood was in the plasma. BAA was the major metabolite of BE in plasma. Ratios of ethylene glycol to BAA in plasma were higher than those in urine. The BE-derived 14C in plasma rapidly became associated with the acid-precipitable (protein) fraction, probably due to binding of metabolites to proteins or incorporation of the BE metabolites into the carbon pool. These results indicate that, in rats, overall metabolism of BE to BAA, the hemolytic metabolite, was linearly related to the exposure concentration up to a concentration that caused severe toxicity (438 ppm). Assuming that the toxicity of inhaled BE is directly proportional to the formation of BAA, the toxicity of inhaled BE can be expected to be linearly related to the exposure concentration up to exposure concentrations that cause mortality.

Metabolism of [14C]benzene by cynomolgus monkeys and chimpanzees.

Rodent bioassays indicate that B6C3F1 mice are more sensitive to the carcinogenicity of benzene than are rats. The urinary profile of benzene metabolites is different in rats vs mice. Mice produce higher proportions of hydroquinone conjugates and muconic acid, indicators of metabolism via pathways leading to putative toxic metabolites, than do rats. In both species, metabolism to hydroquinone and muconic acid is favored at low concentrations of benzene, indicating that these pathways are easily saturated. These species differences in the metabolism of benzene make it difficult to predict the health risk to humans and how this risk varies with dose. For this reason, the metabolism of [14C]benzene by cynomolgus monkeys and chimpanzees, animals phylogenetically closer to humans than rodents, was studied. Monkeys were dosed ip with 5, 50, or 500 mg [14C]benzene/kg body wt. Urine was collected for up to 24 hr following exposure and was analyzed for benzene metabolites. The proportion of the administered 14C excreted in the urine of monkeys decreased from approximately 50 to 15% as the dose increased. Phenyl sulfate was the major urinary metabolite. The proportion of hydroquinone conjugates and muconic acid in the monkey's urine decreased as the dose increased. The proportion of catechol conjugates was not affected by dose. The proportion of these metabolites in the urine was quite variable from animal to animal, but the proportion of muconic acid was consistently much lower in the monkey than in the mouse or rat. Three chimpanzees were administered 1 mg [14C]benzene/kg body wt, iv; essentially all of the injected 14C was recovered in the urine. Of the total urinary metabolites, 79% were accounted for by phenyl conjugates and less than 15% by hydroquinone conjugates or muconic acid. Catechol conjugates were not detected. The metabolism of benzene appeared to be qualitatively similar but quantitatively different in the species studied. The mouse, the sensitive rodent species, forms the highest levels of hydroquinone conjugates and muconic acid and the chimpanzee, the lowest. In all animal species studied for the effect of dose on benzene metabolism, as the dose decreased, a larger proportion of the benzene metabolites was represented by hydroquinone conjugates and muconic acid.

Benzene dosimetry in experimental animals: relevance for risk assessment.

The findings of the studies summarized in this report provide some understanding of the possible role of dosimetry in the different response of the rats and mice to benzene in the long-term bioassay studies. The more sensitive species, the mice, definitely has a higher capacity to metabolize benzene and to metabolize it to more of the putative toxic metabolites than do rats. A major finding of these studies is that in three different animal species, from mice to monkeys, the metabolic pathways leading to production of the putative toxic metabolites appear to be low-capacity, high-affinity pathways that are saturated at relatively low-exposure concentrations. This does not prove, but suggests, that the same may be true in humans. If the total formation of the putative toxic metabolites is predictive of the toxicity of benzene, then the animal studies suggest that calculations of the risk associated with low dose exposures based on the results of animal studies conducted at high doses would underestimate the toxicity of benzene. The current report concerns only dosimetry. Another problem in assessing the risk to humans from benzene exposure is the fact that the animal models do not respond to benzene in the same way as humans. The major concern for humans exposed to benzene, based on epidemiology studies, is the risk of developing acute myelogenous leukemia (Rinksy, 1987). The cancers developed by the rodents on the long-term bioassay studies were at other sites (liver, lung, Zymbal's gland, lymph tissue, ovaries, and mammary gland). There is as yet no good animal model for benzene-induced leukemia. However, it has been suggested that benzene may also increase the incidence of Hodgkin's disease, malignant lymphoma, multiple myeloma and lung cancer in humans, although a statistical basis for this is lacking (Askoy, 1985). It is not unreasonable to assume that whatever form of cancer is induced, the induction is most likely through the reactive metabolites produced from benzene. Therefore, the dosimetry of these metabolites is pertinent. Our studies indicate that benzene metabolite dosimetry data obtained in animals provides data relevant to the estimation of human risks.

Biochemical and morphologic responses of rat nasal epithelia to hyperoxia.

While performing its functions in olfaction, modification of inspired air, and protection of the lower respiratory tract from high concentrations of potentially harmful inhalants, the nasal mucosa can be injured by a number of inhalants. In this study, F344/N male rats were exposed to filtered air or hyperoxia (85 or 87% oxygen), 24 hr/day, 7 days/week, for 1 (acute exposure) or 11 (chronic exposure) weeks. There were distinct differences between the different epithelial regions examined in replicative and morphologic responses as well as altered enzyme activities in response to oxygen exposure. Neither acute nor chronic hyperoxic exposure caused degenerative, necrotizing, or inflammatory changes in any of the nasal epithelial examined. Hyperoxia-induced hypertrophy, but not hyperplasia, of the non-ciliated cuboidal (NCC) epithelium occurred after both acute and chronic exposure. Cell replication was increased in portions of the NCC and respiratory epithelia after acute hyperoxia exposure. There were significant increases, compared to controls, in the specific activity of glucose-6-phosphate dehydrogenase in the nasal turbinates, maxilloturbinates, and lateral wall epithelium (NCC epithelium), the nasal septum (respiratory epithelium), and the ethmoturbinates (olfactory epithelium), and in the specific activity of glutathione peroxidase in the NCC epithelium and ethmoturbinates after acute hyperoxia exposure. The specific activity of cytochrome P450-dependent monooxygenase-catalyzed O-deethylation of 3-cyano-7-ethoxycoumarin was significantly decreased, compared to controls, in the NCC epithelium. These results suggest that hyperoxia exposure induces morphologic and biochemical alterations in nasal epithelia which appear to be protective responses of certain cell types to hyperoxia.

Toxicokinetics of inhaled 1,3-butadiene in monkeys: comparison to toxicokinetics in rats and mice.

1,3-Butadiene is a potent carcinogen in mice and a weaker carcinogen in rats. People are exposed to butadiene through its industrial use--largely in rubber production (over 3 billion pounds of butadiene were produced in 1989)--and because it is common in the environment, occurring in cigarette smoke, gasoline vapor and in the effluents from fossil fuel incineration. Epidemiological studies have provided some evidence for butadiene carcinogenicity in people. Differences in the uptake and metabolism of inhaled butadiene between rodents and primates, including people, might be reflected in differences in its toxicity. In order to compare uptake and metabolism in primates to that in rodents--for which data were already available--we exposed cynomolgus monkeys (Macaca fascicularis) to 14C-labeled butadiene at concentrations of 10.1, 310 or 7760 ppm for 2 hr. Exhaled air and excreta were collected during exposure and for 96 hr after exposure. The uptake of butadiene as a result of metabolism was much lower in monkeys than in rodents. For equivalent inhalation exposures, the concentrations of total butadiene metabolites in the blood were 5-50 times lower in monkey than in the mouse, the more sensitive rodent species, and 4-14 times lower than in the rat. If the toxicokinetics of butadiene in people is more like that of the monkey than that of rodents, then our data suggest that people will receive lower doses of butadiene and its metabolites than rodents following equivalent inhalation exposures to butadiene. This has important implications for assessing the risk to humans of butadiene exposure based on animal studies.

An active feedback system for isotonic studies of smooth muscle.

A feedback system used to perform isotonic studies of smooth muscle is presented. This system is capable of applying a constant force to muscle samples regardless of their contractile activities. The force applied to the tissue is controlled using a proportional integral control system that drives a linear motor. The device is integrated into a sucrose gap tissue bath apparatus where measurements of displacement and electrical activity are also possible. The frequency of canine colonic smooth-muscle electrical oscillations is positively related to applied force.

Effect of repeated benzene inhalation exposures on benzene metabolism, binding to hemoglobin, and induction of micronuclei.

Metabolism of benzene is thought to be necessary to produce the toxic effects, including carcinogenicity, associated with benzene exposure. To extrapolate from the results of rodent studies to potential health risks in man, one must know how benzene metabolism is affected by species, dose, dose rate, and repeated versus single exposures. The purpose of our studies was to determine the effect of repeated inhalation exposures on the metabolism of [14C]benzene by rodents. Benzene metabolism was assessed by characterizing and quantitating urinary metabolites, and by quantitating 14C bound to hemoglobin and micronuclei induction. F344/N rats and B6C3F1 mice were exposed, nose-only, to 600 ppm benzene or to air (control) for 6 hr/day, 5 days/week for 3 weeks. On the last day, both benzene-pretreated and control animals were exposed to 600 ppm, 14C-labeled benzene for 6 hr. Individual benzene metabolites in urine collected for 24 hr after the exposure were analyzed. There was a significant decrease in the respiratory rate of mice (but not rats) pretreated with benzene which resulted in lower levels of urinary [14C]benzene metabolites. The analyses indicated that the only effects of benzene pretreatment on the metabolite profile in rat or mouse urine were a slight shift from glucuronidation to sulfation in mice and a shift from sulfation to glucuronidation in rats. Benzene pretreatment also had no effect, in either species, on formation of [14C]benzene-derived hemoglobin adducts. Mice and rats had similar levels of hemoglobin adduct binding, despite the higher metabolism of benzene by mice. This indicates that hemoglobin adduct formation occurs with higher efficiency in rats. After 1 week of exposure to 600 ppm benzene, the frequency of micronucleated, polychromatic erythrocytes (PCEs) in mice was significantly increased. Exposure to the same level of benzene for an additional 2 weeks did not further increase the frequency of micronuclei in PCEs. These results indicate that repeated exposures to benzene, such as might be encountered by humans as a result of occupational or environmental exposures, are not likely to change or increase benzene metabolism.

Simultaneous measurement of electrical activity from two colonic smooth muscle layers using a dual sucrose gap apparatus.

An apparatus using the sucrose gap technique is presented. With this apparatus simultaneous measurements of contractile and intracellular electrical activity from the two smooth muscle layers of the colon are made. An "L-shaped" muscle preparation consisting of a leg from the circular muscle layer and a leg from the longitudinal muscle layer is used. A theoretical discussion of the device's operation is presented. Finally, experimental results that validate the theory are included.

Electrical and mechanical interactions between the muscle layers of canine proximal colon.

Electrical and mechanical interactions between the two smooth muscle layers of canine colon have been studied using a dual sucrose gap apparatus. Muscle samples were dissected into an L-shape, with one leg cut in the circular direction and the other cut in the longitudinal direction. Longitudinal muscle was removed from the circular leg and circular muscle was removed from the longitudinal leg. The bend of the L contained both layers. The activity of the two layers was studied simultaneously under basal conditions, after stimulation by neostigmine and carbachol, and in the presence of tetrodotoxin. Interactions were more common after stimulation and were marked by modification of one layer's mechanical and electrical activity during increased activity in the other layer. Two patterns were commonly observed. First, during a burst of membrane potential oscillations and spike potentials in the longitudinal layer, slow waves in the circular layer developed spike potentials and some slow waves were also prolonged. Second, during a slow-wave cycle in the circular layer, the amplitude of membrane potential oscillations in the longitudinal layer was increased with an associated increase in the incidence of spike potentials. These interactions were associated with contractions of increased strength, which were similar in both layers. All interactions continued after nerve-conduction blockade by tetrodotoxin.

Disposition of three glycol ethers administered in drinking water to male F344/N rats.

The glycol ethers 2-methoxyethanol (ME), 2-ethoxyethanol (EE), and 2-butoxyethanol (BE) are widely used solvents in industrial and consumer applications. The reproductive, teratogenic, and hematotoxic effects of the glycol ethers are due to the alkoxyacetic acid metabolites of these compounds. The effect of alkyl group length on disposition of these three glycol ethers was studied in male F344/N rats allowed access for 24 hr to 2-butoxy[U-14C]ethanol, 2-ethoxy[U-14C]ethanol, or 2-methoxy[U-14C]ethanol in drinking water at three doses (180 to 2590 ppm), resulting in absorbed doses ranging from 100 to 1450 mumols/kg body wt. Elimination of radioactivity was monitored for 72 hr. The majority of the 14C was excreted in urine or exhaled as CO2. Less than 5% of the dose was exhaled as unmetabolized glycol ether. Distinct differences in the metabolism of the glycol ethers as a function of alkyl chain length were noted. For BE 50-60% of the dose was eliminated in the urine as butoxyacetic acid and 8-10% as CO2; for EE 25-40% was eliminated as ethoxyacetic acid and 20% as CO2; for ME 34% was eliminated as methoxyacetic acid and 10-30% as CO2. Ethylene glycol, a previously unreported metabolite of these glycol ethers, was excreted in urine, representing approximately 10, 18, and 21% of the dose for BE, EE, and ME, respectively. Thus, for longer alkyl chain lengths, a smaller fraction of the administered glycol ether was metabolized to ethylene glycol and CO2. Formation of ethylene glycol suggests that dealkylation of the glycol ethers occurs prior to oxidation to alkoxyacetic acid and, as such, represents an alternate pathway in the metabolism of these compounds that does not involve formation of the toxic acid metabolite.