Mars Extant Life: What's Next? Conference Report.

On November 5-8, 2019, the "Mars Extant Life: What's Next?" conference was convened in Carlsbad, New Mexico. The conference gathered a community of actively publishing experts in disciplines related to habitability and astrobiology. Primary conclusions are as follows: A significant subset of conference attendees concluded that there is a realistic possibility that Mars hosts indigenous microbial life. A powerful theme that permeated the conference is that the key to the search for martian extant life lies in identifying and exploring refugia ("oases"), where conditions are either permanently or episodically significantly more hospitable than average. Based on our existing knowledge of Mars, conference participants highlighted four potential martian refugium (not listed in priority order): Caves, Deep Subsurface, Ices, and Salts. The conference group did not attempt to reach a consensus prioritization of these candidate environments, but instead felt that a defensible prioritization would require a future competitive process. Within the context of these candidate environments, we identified a variety of geological search strategies that could narrow the search space. Additionally, we summarized a number of measurement techniques that could be used to detect evidence of extant life (if present). Again, it was not within the scope of the conference to prioritize these measurement techniques-that is best left for the competitive process. We specifically note that the number and sensitivity of detection methods that could be implemented if samples were returned to Earth greatly exceed the methodologies that could be used at Mars. Finally, important lessons to guide extant life search processes can be derived both from experiments carried out in terrestrial laboratories and analog field sites and from theoretical modeling.

Immunocontraception in Eastern gray squirrels (Sciurus carolinensis): morphologic changes in reproductive organs.

Eastern gray squirrels (EGS) (Sciurus carolinensis) damage trees through bark stripping or gnawing due to territorial marking or agonistic gnawing behavior in concert with higher densities. This study was conducted to determine the effects of a contraceptive vaccine on EGS and its reproductive organ histology. Free-ranging urban EGS were vaccinated with the immunocontraceptive GonaCon. All EGS were > or = 6 mo of age as determined by a combination of pelage characteristics and body weights. The vaccine was administered by injection at a dosage rate of 0.4 ml containing 400 microg of GnRH-mollusk protein conjugate i.m. in the thigh to 33 EGS (17 male [m], 16 female [f]) in trapping session 1 (TS1), 23 (14 m, 9 f) in trapping session 2 (TS2), and 11 (8 m, 3 f) in trapping session 3 (TS3). A sham injection containing 0.4 ml saline-AdjuVac was given as control to 22 EGS (16 m, 6 f) in TS1, 20 (12 m, 8 f) in TS2, and 8 (4 m, 4 f) in TS3. In the last trapping session (TS4), 35 EGS (16 treated, 19 control) were killed for necropsy to evaluate histologic changes in testes and ovaries. Treated EGS males had testicular, prostatic, and epididymal atrophy compared with control EGS males. The tubuli seminiferi and prostatic glandular lumen of treated EGS males were atrophic, and the epididymal lumen contained no sperm cells. No histologic changes were observed in treated EGS females; however, females likely were not collected when changes due to GonaCon would have been observed. There were no observable histologic differences in the pituitary gland of treated and control EGS. There were no statistically significant differences in either testosterone or progesterone concentrations between control and treated EGS. Although there were no serious side effects to the vaccine, six EGS developed injection site abscesses. GonaCon may be a potential tool for EGS population control.

Class II correction in transitional dentition.

Correcting the developing Class II malocclusion and preventing abnormal relationship of the jaws and occlusal dysfunction by repositioning the permanent molars and incisors is a significant benefit to the growing child. Future orthodontic therapy is significantly reduced often to detailing or eliminated in some cases.

Preclinical safety evaluation of rhuMAbVEGF, an antiangiogenic humanized monoclonal antibody.

Recombinant humanized antivascular endothelial growth factor (rhuMAbVEGF) is a monoclonal IgG1 antibody that is being developed as an antiangiogenic agent for use in treating a variety of solid tumors. Preclinical safety studies included an immunohistochemical tissue cross-reactivity study, in vitro hemolytic potential and blood compatibility studies, and multiple dose toxicity studies. Toxicity studies were conducted in cynomolgus monkey because rhuMAbVEGF is pharmacologically active in this species and does not bind rat or mouse vascular endothelial growth factor (VEGF). Following twice weekly administration of rhuMAbVEGF for 4 or 13 wk, young adult cynomolgus monkeys exhibited physeal dysplasia characterized by a dose-related increase in hypertrophied chondrocytes, subchondral bony plate formation, and inhibition of vascular invasion of the growth plate. In addition, decreased ovarian and uterine weights and an absence of corpora lutea were observed in females receiving 10 and 50 mg/kg/dose in the 13-wk study. Both the physeal and ovarian changes were reversible with cessation of treatment. No other treatment-related effects were observed following rhuMAbVEGF administration at doses up to 50 mg/kg. These findings indicate that VEGF is required for longitudinal bone growth and corpora lutea formation and that rhuMAbVEGF can reversibly inhibit physiologic neovascularization at these sites.

Skin carcinogenicity of condensed asphalt roofing fumes and their fractions following dermal application to mice.

Condensed roofing asphalt fumes, generated at 316 degrees C, were collected by cold trap condensation and fractionated by preparative high performance liquid chromatography. Chemical classes in each of the fractions (A-E) were identified by gas chromatography/mass spectroscopy. The fractions, various combinations of fractions, the raw and heated asphalt, the neat asphalt fume and the reconstituted asphalt were tested for carcinogenicity, and three fractions were tested for cocarcinogenicity and tumor promotion with benzo[a]pyrene (BaP). The skin application carcinogenesis bioassay was conducted by twice weekly application of test materials in 0.05 ml of acetone/cyclohexane (1:1) for 104 weeks to 40 groups of male C3H/HeJ mice (30/group). Fractions were applied at a mass in proportion to their amount in the neat asphalt fumes. In addition, the neat asphalt fume was tested on Sencar mice to determine if this strain was more susceptible to the carcinogenic effects of the fumes. Condensed neat asphalt fumes produced similar and statistically significant increased tumor yields of papillomas and carcinomas in both strains as compared to respective vehicle controls. Recombination of all fractions resulted in a tumor response similar to neat asphalt fumes. Among individual fractions, C was most potent, followed by B. The other single fractions were without significant tumorigenic activity. Combinations containing fractions B and C were most active among the mixtures that were assayed and no evidence of enhancement of tumorigenesis in the mixtures was found. No significant cocarcinogenic or tumor promoting activity was observed with fractions A, D, or E and BaP. Raw unheated asphalt produced a few tumors in C3H mice, but no tumors were seen when raw asphalt heated to 316 degrees C, with the fumes permitted to escape, was applied.

Comparison of the toxicity profiles of ISIS 1082 and ISIS 2105, phosphorothioate oligonucleotides, following subacute intradermal administration in Sprague-Dawley rats.

The systemic toxicity of two phosphorothioate oligonucleotides specific for herpes simplex viruses (ISIS 1082) and human papiloma virus (ISIS 2105) were evaluated following repeated intradermal injections of vehicle control, 0.33, 2.17, or 21.7 mg/kg daily to Sprague-Dawley rats (10/sex/group) for 14 days. Animals were sacrificed 1 day after the last dose, except for a portion of the ISIS 1082-treated animals (5/sex/group) which were maintained for an additional 14-day recovery period. The profile of alterations noted for both compounds was very similar. Other than local signs of irritation at the site of injection, there were no clinical signs of toxicity or treatment-related mortality, but there was a slight decrease in body weight gain for the 21.7 mg/kg dose groups. Alterations in hematology parameters included dose-dependent thrombocytopenia and anemia. Alterations in serum chemistry parameters were suggestive of mild alterations in hepatic metabolism, with increases in liver transaminases and bilirubin, along with decreases in albumin and cholesterol. Both spleen and liver weights were significantly elevated in a dose-dependent fashion. Histopathological alterations noted in liver, kidney, lung, injection site skin, and spleen were characterized as perivascular and interstitial infiltrates of macrophages and monocytes. Additional microscopic alterations in the spleen included mild lymphoid hyperplasia (seen in lymph nodes as well), and extramedullary hematopoiesis. Treatment-related cytopenias were likely related to mild, focal hypocellularity in the bone marrow. Alterations in ISIS 1082-treated animals were only partially reversed following the 14-day treatment-free period. In conclusion, repeated intradermal administration of ISIS 1082 and ISIS 2105 produced a similar spectrum of toxicities, with liver, kidney, spleen, and bone marrow being identified as target tissues.

Influence of dietary protein, fat, and fiber on growth, blood chemistry, and tumor incidences in Fischer 344 rats.

Composition of diets may influence growth, diseases, tumor rates, and responses to chemical treatment. For two years, Fischer 344 rats were fed the NIH-07 open-formula nonpurified diet (approximately 23% protein, approximately 5% fat, and approximately 3.5% fiber) and nonpurified experimental diets (NTP-90, NTP-91, and NTP-92) containing lower protein and higher fat and fiber (14.6-15.3% protein, 7.2-8.5% fat, and 9.4-14% fiber) than the NIH-07 diet. Rats were evaluated for growth patterns, survival, hematology, serum chemistry, nephropathy, and tumor incidences. Growth patterns were similar in rats fed the experimental diets and in those fed the NIH-07 diet. However, in rats fed the experimental diets, the adult body weights were significantly (6-9%) lower and the survival at 110 weeks of age was significantly higher (15-20%) than in rats fed the NIH-07 diet. Lower protein content of experimental diets decreased the severity of nephropathy. Higher fat content of experimental diets appears to have decreased the incidence or delayed the development of leukemia and associated mortality in males. Higher fiber content of experimental diets appears to have delayed the development of mammary tumors and associated mortality in females. Higher fat and/or fiber of the experimental diets decreased the incidence of pheochromocytomas in males. The lower protein and higher fat and fiber contents of the experimental diets decreased the spontaneous tumor burden in two-year studies. These studies indicate that diets for rats in long-term studies could be modified to decrease the severity of nephropathy and to decrease/delay the development of spontaneous tumors.

Effects of exercise training on absolute and relative measurements of regional adiposity.

The purpose of the present study was to determine whether absolute and relative measurements of regional adiposity differ in their responses to exercise intervention and which measures are most predictive of changes in plasma lipids, insulin sensitivity and adiposity. Thirteen middle-aged men (BMI 30.4 +/- 1.5 kg/m2, age 47.2 +/- 1.5 years, mean +/- s.e.) were examined before and after 14 weeks of endurance-oriented physical activity (3-4 days/week, 30-45 min/day). Significant (P < 0.05) decreases in the absolute measures of chest, waist and hip girths and sagittal diameter were evident. The waist-to-hip ratios (WHR) of umbilicus/maximal hip and minimal waist/maximal hip decreased significantly (P < 0.05). However, the WHRs of umbilicus/anterior superior iliac spine and umbilicus/greater trochanters did not change due to parallel decreases in waist and hip girths. Trunk and extremity skinfolds decreased significantly (P < 0.05); however, trunk/extremity skinfold ratios were virtually unaltered. The training programme significantly (P < 0.05) increased insulin sensitivity (60%) and HDL (8%), and reduced triglyceride (25%) and total cholesterol/HDL (8%). Changes in these variables were related to changes in sagittal diameter and waist girth. These data indicate different responses to physical activity between measurements of regional adiposity, and emphasize the need for considering absolute central girths such as waist circumference and sagittal diameter when assessing fat topography and cardiovascular risk.

Effects of exercise training on the chemical composition of plasma LDL.

The purpose of the present study was to determine the effects of exercise training on the chemical composition of plasma low-density lipoprotein (LDL). Thirteen men (mean age +/- SE, 47.2 +/- 1.5 years) were examined before and after 14 weeks of endurance-oriented exercise training (3 to 4 d/wk, 30 to 45 min/d). Although calculated plasma LDL concentrations remained unaltered (3.49 +/- 0.24 versus 3.65 +/- 0.23 mmol/L), changes in the chemical composition of LDL (increased LDL free cholesterol, cholesterol ester, and phospholipid content) were associated with a reduction in adiposity, umbilical girth, and basal plasma insulin and glucose concentration with training intervention. Increases in LDL molecular weight and particle diameter were associated with a reduction in fat mass, plasma triglyceride concentration, and basal plasma glucose concentration with physical activity. The LDL lipid-to-protein ratio also increased (P < .01) with training by 7%, primarily due to an increase in LDL free cholesterol content (P < .01). These findings indicate the formation of LDL particles that are more cholesterol enriched and protein poor with exercise training, which provides additional evidence for the cardioprotective effect of long-term physical activity.

Exercise training increases GLUT-4 protein concentration in previously sedentary middle-aged men.

The purpose of this study was to determine if 14 wk of exercise training would increase insulin-sensitive glucose transporter protein (GLUT-4) concentration in skeletal muscle of previously sedentary middle-aged men (47.2 +/- 1.3 yr; n = 13). Muscle samples (lateral gastrocnemius) and insulin action [insulin sensitivity index (ISI), minimal model] were obtained in the sedentary condition and 48 h after the final training bout. GLUT-4 protein concentration increased (P < 0.001, 2,629 +/- 331 to 4,140 +/- 391 absorbance units/100 micrograms protein) with exercise training by 1.8-fold. ISI increased by twofold (P < 0.05, 2.1 +/- 0.5 to 3.4 +/- 0.7 SI x 10(5) min/pM) with training. The percentage of GLUT-4 rich type IIa muscle fibers increased by approximately 10% (P < 0.01), which may have contributed to the elevation in transporter protein. GLUT-4 concentration and citrate synthase activity (1.7-fold, P < 0.001) also increased by similar increments. These findings indicate that GLUT-4 protein concentration is elevated in middle-aged individuals with exercise training.

The toxicologic and oncogenic potential of JP-4 jet fuel vapors in rats and mice: 12-month intermittent inhalation exposures.

Three-hundred Fischer 344 rats and 300 C57BL/6 mice of each sex were divided into three treatment groups and exposed intermittently (6 hr/day, 5 days/week) to JP-4 jet fuel vapors at concentrations of 0, 1000, and 5000 mg/m3 for 12 months. At exposure termination, 10% of the animals were killed and those remaining were held for a 12-month postexposure tumorigenesis observation period. Pathologic findings in male rats revealed treatment-related renal toxicity and neoplasia consistent with the male rat unique alpha 2 mu-globulin nephropathy syndrome. Distinct JP-4-induced respiratory toxicity was not observed, and pulmonary neoplasms were not significantly increased in any treatment group. Benign hepatocellular adenomas were slightly increased in high-dose female mice, but the trend was reversed in male mice. Other pathologic findings were regarded as equivocal or compatible with expected biologic variation. The study did not demonstrate target organ toxicity or carcinogenesis which could be extrapolated to other species.

Spontaneous corneal dystrophy and generalized basement membrane changes in Fischer-344 rats.

Groups of young, sexually mature Fischer-344 rats (n = 25/sex) obtained from commercial breeders were examined ophthalmologically and histopathologically to determine the prevalence and severity of corneal basement membrane lesions (corneal dystrophy) and basement membrane changes in select nonocular tissues. Results disclosed a high incidence of corneal basement membrane dystrophy in rats of both sexes from all breeders; however, severity levels were significantly increased in rats obtained from one breeder when compared to others. Furthermore, rats that displayed the most advanced corneal lesions also exhibited more severe basement membrane changes in other organs, especially renal tubules and vascular internal laminae. These findings suggest that both ocular and nonocular dystrophic changes may have been linked through common physiologic (or genetic) mechanisms. Animals that displayed basement membrane lesions were not considered to represent compromised biologic test systems.

The pathology of chronic bovine fluorosis: a review.

Clinical, pathologic, and analytical records from 200 cattle were reviewed to determine if long-term exposures to elevated fluorides resulted in previously unrecognized or unreported pathologic changes, especially skeletal neoplasia. Animals were part of comprehensive field and laboratory investigations of bovine fluorosis conducted by the Utah State University Agricultural Experiment Station over a 25-year period. Records indicated that over 170 cattle included in this review were exposed to dietary fluorides levels in excess of 25 ppm (dry wt), for most of their life span, and these animals exhibited bone fluoride concentrations ranging between 2,000 and 12,500 ppm (dry wt). Although dental and/or skeletal changes were present in most animals, significant soft tissue damage or neoplasia was not observed in any organ system. Renal degeneration and mineralization were slightly more prevalent in range cattle ingesting high fluoride levels, but these changes were not recognized in animals that received high experimental fluoride doses. The absence of significant soft tissue damage or neoplasia in these cattle combined with results of an extensive literature review suggests that environmental fluorides are not significant factors in mammalian carcinogenesis.

Histopathology in hosts parasitized by Sarcoptes scabiei.

Histopathologic evaluation of nondermal tissue in rabbits infested with Sarcoptes scabiei var. canis was investigated. Severe infestation resulted in deviant serological and serum biochemical values. Histological study revealed structural changes in the tissues of specific organs. The most prominent histological finding was amyloidosis in the liver, glomerulus of the kidney, red pulp of the spleen, intestines, and tongue. Hosts treated for infestation exhibited no abnormal organ histology.

The determination of the repeated oral toxicity of halocarbon oil, series 27-S.

Halocarbon 27-S (HC 27-S), a polymer of chlorotrifluoroethylene (CTFE), is used as a lubricating oil for pumps in hyperbaric chambers. Although monomeric CTFE has been shown to produce renal lesions in rats, the toxicity of CTFE polymers have not been investigated. To assess the toxicity of repeated exposure to HC 27-S, three groups (N = 6/group) of male and female Fischer-344 rats were dosed with 2.5 g HC 27-S/kg for 7 or 21 consecutive days. Groups were sacrificed at 7, 21, and 35 days (14 days after the 21-day dosing). Corresponding control groups (N = 6) were dosed with deionized water. Decreased water consumption and urine output were apparent in all test groups. Statistically significant increases in fluoride excretion were noted in 24-hr urine samples assessed periodically during the study. Neurotoxic signs were observed in female rats but not in male rats. Significant increases in liver and kidney weights were seen in all rats, regardless of number of dosing days. The increased fluoride burden in treated animals appeared sufficient to alter bone calcium/phosphate ratios in male rats but not female rats. Gross liver enlargement and hepatocellular cytomegaly indicated that the liver was probably the primary target organ following repeated administration of HC 27-S.

Tropism and histopathology of the D, B, K, and MM variants of encephalomyocarditis virus.

Variants of encephalomyocarditis virus (EMCV), which are immunologically indistinguishable by hyperimmune serum, produce different disease syndromes in mice. For instance, in ICR Swiss male mice, EMCV-B produces no overt illness, EMCV-MM produces severe neurological signs followed by death, EMCV-D destroys pancreatic beta cells producing a disease syndrome resembling insulin-dependent diabetes mellitus, and EMCV-K is lethal but produces no overt signs of infection. The present study was done to determine the tissue tropism and histopathology of each of these EMCV variants in the ICR Swiss mouse model. The data show the highest concentrations in the following organs: EMCV-D in the pancreas, EMCV-B in the pancreas, EMCV-MM in the cerebrum, and EMCV-K in the medulla/brainstem. They also show that the pathological lesions produced by each variant correlate well with viral titers.

Blood pressure and hepatocellular effects of the cyclic heptapeptide toxin produced by the freshwater cyanobacterium (blue-green alga) Microcystis aeruginosa strain PCC-7820.

Laboratory rats and mice were used to investigate the hepatotoxicity caused by the cyclic heptapeptide (mol. wt 994) termed microcystin-LR. Microcystin-LR (also known as cyanoginosin-LR) is produced by the freshwater cyanobacterium (blue-green alga) M. aeruginosa strain PCC-7820. In time course histopathology studies with mice significant liver damage, with an absence of pulmonary emboli, were observed after 15 min. Pulmonary emboli did not appear until 1 hr. In rats, significant liver damage and the presence of occasional emboli were observed at 20 min. Pulmonary emboli did not contain fibrin nor appear life-threatening in any case and resembled the globular eosinophilic debris found in the liver sinusoids and central veins. Measurements of rat femoral arterial, jugular venous and hepatic portal venous blood pressures during the course of toxicity revealed a slowly declining arterial pressure and stable, normal venous pressures. Blood lactic acid levels rose in parallel with the fall in arterial pressure, a pattern typical of hemorrhagic shock. There was no indication of venous congestion that would accompany right heart failure. Isolated, perfused rat livers dosed with toxin showed rapid changes in the liver, including cessation of bile flow within 10 min and complete obliteration of normal lobular architecture within 60 min. No effect of the toxin was observed in isolated perfused rat heart. We conclude that in the mouse and rat, microcystin-LR is a potent, rapid-acting, direct hepatotoxin, with the immediate cause of death in acute toxicities being hemorrhagic shock secondary to massive hepatocellular necrosis and collapse of hepatic parenchyma.

Pathological and hepatic ultrastructural effects of a single dose of perfluoro-n-decanoic acid in the rat, hamster, mouse, and guinea pig.

In rats, the liver is the primary target organ of perfluoro-n-decanoic acid (PFDA) toxicity. Therefore, the effects of PFDA on hepatic ultrastructure were studied in rats. Pathological changes induced by PFDA in hamsters, mice, and guinea pigs were also examined. PFDA caused a severe reduction in body weight in all four species studied. A reduction in food intake was observed in rats and hamsters. However, hamsters continued to consume food at a reduced level, while rats stopped eating for a 5- to 6-day period about 6 days after dosing. The PFDA-induced pathological changes in the hamsters, mice, and guinea pigs resembled those seen in rats to varying degrees. As in the rat, PFDA caused a marked liver enlargement in mice and hamsters and a moderate swelling in guinea pigs. This hepatomegaly was ascribed primarily to individual cell swelling. Thymic atrophy was noted in PFDA-treated hamsters, mice, and guinea pigs. Seminiferous tubular degeneration observed in hamsters and guinea pigs, but not in mice, was not as severe as in the rat, where in some cases frank necrosis has been seen. Ultrastructural changes in the livers of all PFDA-treated animals, regardless of species, included disruption of the rough endoplasmic reticulum, rounding and swelling of the mitochondria with related structural alterations, and mild to extensive proliferation of peroxisomes. This peroxisome proliferative response was greatest in mice and almost absent in guinea pigs. Accumulation of lipid droplets in liver cells due to PFDA treatment was more pronounced in hamsters and guinea pigs than in rats and mice. PFDA-induced hepatomegaly with a concomitant increase in peroxisomes in several rodent species may be associated with an impairment of normal lipid metabolism in the liver by PFDA.

A subchronic dermal exposure study of diethylene glycol monomethyl ether and ethylene glycol monomethyl ether in the male guinea pig.

Diethylene glycol monomethyl ether (DEGME) has been selected as a replacement anti-icing additive for ethylene glycol monomethyl ether (EGME) in Navy jet aircraft fuel. This experiment was performed to determine whether DEGME produced similar toxicity to EGME following dermal exposure. Male guinea pigs were dermally exposed to 1.00, 0.20, 0.04, or 0 (control) g/kg/day DEGME for 13 weeks, 5 days/week, 6 hr/day. Another group of animals was similarly exposed to 1.00 g/kg/day EGME. Body weights as well as testicular and splenic weights were reduced as a result of exposure to EGME, DEGME-exposed animals exhibited decreased splenic weight in the high- and medium-dose (1.00 and 0.20 g/kg/day) exposure groups only. Hematologic changes in EGME-exposed animals included mild anemia with increased erythrocytic mean corpuscular volumes and a lymphopenia with increased neutrophils. Similar hematological changes were not observed in any animals exposed to DEGME. Serum creatine kinase activity was increased in animals exposed to EGME, and serum lactate dehydrogenase activity was increased in EGME and 1.00 g/kg/day DEGME-exposed animals. In general, DEGME produced minimal toxicological changes following dermal exposure, whereas the toxicological changes observed following similar exposure to EGME were much more profound.

Long-term inhalation toxicity of hydrazine.

Year-long intermittent exposures of rats, mice, hamsters, and dogs to hydrazine were conducted using concentrations of 0.05, 0.25, 1.0, and 5.0 ppm. Rats were held 18 months postexposure; hamsters, 1 year postexposure; mice, 15 months postexposure; and dogs, 38 months postexposure. Male and female rats exhibited dose-dependent incidences of benign nasal adenomatous polyps and smaller numbers of malignant nasal epithelial tumors after 1 year of exposure to hydrazine and 18 months postexposure holding. Nasal tumors were often associated with chronic irritation and were most frequent in male rats, with an incidence of greater than 50% in the highest exposure group. Hamsters exposed to 0.25-ppm and higher concentrations showed pathologic changes characteristic of degenerative disease, including amyloidosis. After exposure to 0.5 ppm hydrazine, hamsters developed a 10% incidence of benign nasal polyps compared to 0.5% in controls. Small numbers of colon neoplasms and thyroid parafollicular cell adenomas were found in hamsters, but only in the highest concentrations tested. Lung adenomas appeared to be marginally increased in mice exposed to 1.0 ppm hydrazine, the highest concentration tested in this species. No consistent clinical or pathological effects were seen in dogs during or after exposure to hydrazine at any concentration. Using amyloidosis as a criterion, a no-effect level was not achieved in hamsters. In rats, there appeared to be a marginal production of nasal tumors at 0.05 ppm, while mice showed no effects at 0.25 ppm. This study has demonstrated that the nasal respiratory epithelia of rats and hamsters are the most sensitive tissues to the tumorigenic action of hydrazine following inhalation exposures. This is similar to the reaction of rats to formaldehyde, another highly reactive water-soluble compound.