Determination of flumequine in channel catfish by liquid chromatography with fluorescence detection.

Rapid methods are described for determination of flumequine (FLU) residues in muscle and plasma of farm-raised channel catfish (Ictalurus punctatus). FLU residues were extracted from tissues with an acidified methanol solution, and extracts were cleaned up on C18 solid-phase extraction cartridges. FLU concentrations were determined by liquid chromatography (LC) using a C18 analytical column and fluorescence detection (excitation, 325 nm; emission, 360 nm). Mean recoveries of FLU from fortified muscle were 87-94% at 5 levels ranging from 10 to 160 ppb (5 replicates per level). FLU recoveries from fortified plasma were 92-97% at 5 levels ranging from 20 to 320 ppb. Limits of detection (signal-to-noise ratio, 3:1) for the method as described were 3 and 6 ppb for muscle and plasma, respectively. Relative standard deviations (RSDs) for recoveries were < or = 12%. Live catfish were dosed with 14C-labeled or unlabeled FLU to generate incurred residues. Recoveries of 14C residues throughout extraction and cleanup were 90 and 94% for muscle and plasma, respectively. RSDs for incurred FLU at 2 levels in muscle and plasma ranged from 2 to 6%. The identity of FLU in incurred tissues was confirmed by LC/mass spectrometry.

Analysis of malachite green and metabolites in fish using liquid chromatography atmospheric pressure chemical ionization mass spectrometry.

Malachite green (MG), a traditional agent used in aquaculture, is structurally related to other carcinogenic triphenylmethane dyes. Although MG is not approved for use in aquaculture, its low cost and high efficacy make illicit use likely. We developed sensitive and specific methods for determination of MG and its principal metabolite, leucoMG (LMG), in edible fish tissues using isotope dilution liquid chromatography atmosphere pressure chemical ionization mass spectrometry. MG and LMG concentrations were measured in filets from catfish treated with MG under putative use conditions (ca. 250 and 1000 ppb, respectively) and from commercial trout samples (0-3 and 0-96 ppb, respectively). Concentrations of LMG in edible fish tissues always exceeded those of MG. A rapid cone voltage switching acquisition procedure was used to simultaneously produce molecular ions for quantification and diagnostic fragment ions for confirmation of MG and metabolites. The accurate and precise agreement between diagnostic ion intensity ratios produced by LMG in authentic standards and incurred fish samples was used to unambiguously confirm the presence of LMG in edible fish tissue. This suggested the validity of using LMG as a marker residue for regulatory determination of MG misuse. Additional metabolites derived from oxidative metabolism of MG or LMG (demethylation and N-oxygenation) were identified in catfish and trout filets, including a primary arylamine which is structurally related to known carcinogens. The ability to simultaneously quantify residues of MG and LMG, and to confirm the chemical structure of a marker residue by using LC/MS, suggests that this procedure may be useful in monitoring the food supply for the unauthorized use of MG in aquaculture.

Liquid chromatographic determination of flumequine, nalidixic acid, oxolinic acid, and piromidic acid residues in catfish (Ictalurus punctatus).

A peer-verified, liquid chromatographic (LC) method for simultaneous determination of residues of flumequine (FLU), nalidixic acid (NAL), oxolinic acid (OXO), and piromidic acid (PIR) in catfish muscle is presented. Sample workup involves homogenizing tissue with acetone, defatting with hexane, and extracting quinolones into chloroform. Sample is purified further by partitioning into base and then subsequently back-extracting into chloroform after acidifying the aqueous phase. After solvent is evaporated, the residue is diluted with mobile phase, and analytes are introduced into an LC system where separations are made with a 5 microns, reversed-phase polymer column and an isocratic, buffered acetonitrile-tetrahydrofuran mobile phase. Determinations are made by UV detection at 280 nm for PIR and by fluorescence detection (excitation at 325 excitation and emission at 365 nm) for the other 3 analytes. Each quinolone was used to fortify catfish muscle at 5, 10, 20, 40, and 80 ng/g. The following recoveries and relative standard deviation (RSD) values represent an average of the 5 levels for each analyte: FLU, 79.7% (RSD = 5.7%); OXO, 80.8% (RSD = 6.3%); PIR, 75.0% (RSD = 5.9%); and NAL, 87.1% (RSD = 10%). Assay of 5 levels (base incurred catfish, plus 4 dilutions with control catfish) of catfish muscle incurred with the 4 quinolones gave the following averages: FLU: base, 198 ng/g (RSD = 2.3%); dilutions, 98.0 ng/g (RSD = 4.2%), 61.6 ng/g (RSD = 4.4%), 21.6 ng/g (RSD = 2.8%), 9.24 ng/g (RSD = 8.7%); OXO, base, 257 ng/g (RSD = 6.9%); dilutions, 146 ng/g (RSD = 5.5%), 95.0 ng/g (RSD = 4.1%), 30.7 ng/g (RSD = 3.8%), 13.7 ng/g (RSD = 4.6%); PIR, base, 22.1 ng/g (RSD = 4.2%); dilutions, 13.7% ng/g (RSD = 6.7%), 6.49 ng/g (RSD = 15%), 2.65 ng/g (RSD = 15%); and NAL, base, 75.1 ng/g (RSD = 3.8%); dilutions, 42.3 ng/g (RSD = 5.1%), 24.1 ng/g (RSD = 6.3%), 8.59 ng/g (RSD = 4.8%). A second multiresidue analysis of the 4 quinolones was performed by an outside analyst. Average recoveries from catfish fortified at 5, 10, 20, and 40 ng/g were FLU, 75.9% (RSD = 4.0%); OXO, 84.0% (RSD = 5.5%); NAL, 85.6% (RSD = 8.9%); and PIR, 66.2% (RSD = 8.7%).

Pharmacokinetics, tissue distribution and metabolism of acriflavine and proflavine in the channel catfish (Ictalurus punctatus).

1. The disposition of proflavine (PRO) and acriflavine (ACR) were examined in channel catfish after intravascular (i.v.) dosing (1 mg/kg) or waterborne exposure (10 mg/l for 4 h). 2. After i.v. dosing, plasma concentration-time profiles of parent PRO and ACR were best described by two- and three-compartment pharmacokinetic models respectively. Terminal elimination half-lives of PRO and ACR in plasma were 8.7 and 11.4 h respectively. 3. In animals dosed with 14C-PRO or 14C-ACR, total drug equivalent concentrations were highest in the excretory organs and lowest in muscle, fat and plasma. In PRO-dosed animals, residues in the liver and trunk kidney were composed primarily of glucuronosyl and acetyl conjugates of PRO; residues in muscle were composed mostly (> 95%) of the parent drug. In ACR-dosed animals, the parent compound comprised > 90% of the total residues in all tissues examined. 4. PRO and ACR were poorly absorbed in catfish during waterborne exposure. At the end of a 4-h exposure, parent PRO and ACR concentrations in muscle were 0.064 and 0.020 microgram/g respectively. Levels in muscle declined below the limit of determination (0.005 microgram/g) within 1-2 weeks.

Liquid chromatographic determination of acriflavine and proflavine residues in channel catfish muscle.

A liquid chromatographic (LC) method was developed for determination of acriflavine (ACR) and proflavine (PRO) residues in channel catfish muscle. Residues were extracted with acidified methanol solution, and extracts were cleaned up with C18 solid-phase extraction columns. Residue concentrations were determined on an LC cyano column, with spectrophotometric detection at 454 nm. Catfish muscle was individually fortified with ACR (purified from commercial product) and PRO at concentrations of 5, 10, 20, 40, and 80 ppb (5 replicates per level). Mean recoveries from fortified muscle at each level ranged from 86 to 95%, with relative standard deviations (RSDs) of 2.5 to 5.7%. The method was applied to incurred residues of ACR and PRO in muscle after waterborne exposure of channel catfish to commercial acriflavine (10 ppm total dye for 4 h). RSDs for incurred residues of ACR and PRO were in the same range as those for fortified muscle. Low residue concentrations (< 1% of exposure water concentration) suggested poor absorption of ACR and PRO in catfish.

Determination of methylene blue in channel catfish (Ictalurus punctatus) tissue by liquid chromatography with visible detection.

Methylene blue (MB) is a thiazine dye that, although not regulated for use with edible fish, may sometimes be used as a chemotherapeutic agent in the aquaculture industry. A liquid chromatographic (LC) method was developed for the determination of MB in fish muscle. MB was extracted from catfish tissue with an acetonitrile-acetate buffer solution containing hydroxylamine and toluenesulfonic acid, partitioned into methylene chloride, and cleaned up on alumina and carboxylic acid solid-phase extraction cartridges. MB concentrations were determined by LC with visible detection at 660-665 nm. Recoveries of MB from catfish fortified at 10-50 ng/g were 75-90% (RSDs, 5-12%). Leucomethylene blue also can be recovered from catfish tissue as the MB colored form at low levels with similar recoveries. Analysis of catfish exposed to MB in a bath treatment at 5 ppm MB for 1 h recovered 10-20 ppb of the drug in the muscle tissue. The low tissue concentration suggests poor absorption of this drug compared with other antifungal dyes that tend to concentrate and remain in fish tissue at high levels.

Determination and confirmation of identities of flumequine and nalidixic, oxolinic, and piromidic acids in salmon and shrimp.

A previously published liquid chromatographic (LC) method for determining residues of flumequine (FLU) and nalidixic (NAL), oxolinic (OXO), and piromidic (PIR) acids in catfish tissue was applied to salmon and shrimp muscle. Identities of all 4 residues in salmon and shrimp were confirmed by gas chromatography/mass spectrometry (GC/MS). The tissue is homogenized with acetone, the acetone extract is defatted with hexane, and the quinolones are extracted into chloroform. The extract is further purified by first partitioning into base and then back-extracting from a solution acidified to pH 6.0. Analytes are determined by LC with simultaneous UV and fluorescence detection. Muscle tissue was fortified with each quinolone at 5, 10, 20, 40, and 80 ng/g. Average recoveries and relative standard deviations (RSDs) for salmon, which represent an average of the 5 levels for each analyte, ranged from 75.9 to 90.8% and from 2.25 to 6.40%, respectively. Average recoveries and RSDs for shrimp ranged from 81.3 to 91.2% and from 7.34 to 10.7%, respectively. Identities of OXO, FLU, NAL, and PIR were confirmed in extracts of salmon and shrimp tissue fortified at 10 ng/g by determination of decarboxylated quinolones by GC/MS. Four diagnostic ions were monitored for OXO, FLU, and PIR, and 5 ions were monitored for NAL. All ion relative abundances were within 10% of those calculated for standard decarboxylated quinolones. Optimum conditions for decarboxylation and GC/MS confirmation are given.

Optimization of a liquid chromatographic method for determination of malachite green and its metabolites in fish tissues.

A liquid chromatographic (LC) method was adapted and optimized for the determination of malachite green and its metabolites in fish plasma and muscle. Residues in plasma were extracted with acetonitrile, the extract was evaporated to dryness, and residues were resolubilized for LC analysis. Residues in muscle were extracted with an acetonitrile-acetate buffer mixture, reextracted with acetonitrile, and partitioned into methylene chloride with final cleanup on alumni and propylsulfonic acid solid-phase extraction columns. Residue levels were determined by using an LC cyano column with a PbO2 postcolumn and visible detection (618 nm). Overall mean recoveries of parent malachite green (MG-C) and its major metabolite, leuco-malachite green (MG-L), from plasma were 93 and 87%,respectively, at fortification levels ranging from 25 to 250 ppb. Overall mean recoveries of MG-C and MG-L from muscle were 85 and 95%, respectively, at fortification levels ranging from 5 to 100 ppb. Relative standard deviations (RSDs) of recoveries at all fortification levels ranged from 3.9 to 7.0% for plasma and from 2.1 to 5.2% for muscle. The method was applied to incurred residues in tissues sampled from catfish after waterborne exposure to [14C]MG-C. Mean recoveries of total radioactive residues in plasma and muscle throughout the extraction and cleanup process were 88 and 87%, respectively, and corresponding RSDs for MG-C and MG-L were in the same range as those for fortified tissues. MG-L was confirmed as the major metabolite of MG-C in catfish.

Optimization of the analytical performance of the magnetic sector mass spectrometer for the identification of residual chloramphenicol in shrimp.

Chemical noise limits mass spectrometric detection of chloramphenicol (CAP) with electron capture ionization at low resolution, and makes CAP identification at concentrations of 5 parts per billion (ppb) difficult. Increasing the resolution from 1000 to 3500, however, was sufficient to separate the analyte signals from the noise signals, and resulted in a 100 times higher analytical sensitivity. The introduction of sweep gas in the ion source decreased the scattering of the quantitative results on average by a factor of 7, and thereby improved the precision of the analyses to an acceptable level (CV < 10%). Under such conditions, CAP residues of 1.5 and 2.1 ppb in shrimp as determined by electron capture gas chromatography/mass spectrometry can readily be identified by monitoring four diagnostic ions.

Furazolidone disposition after intravascular and oral dosing in the channel catfish.

1. The pharmacokinetics, tissue distribution and excretion of the nitrofuran drug furazolidone have been examined in the channel catfish. [14C]Furazolidone was administered by intravascular or oral routes in a single dosage of 1 mg/kg body weight. 2. A two-compartment pharmacokinetic model best described parent furazolidone concentrations in the plasma after intravascular dosing. Elimination of parent compound was extremely rapid, with a terminal half-life of 0.27h and total body clearance of 1901 ml/h/kg. 3. After oral dosing, furazolidone concentrations in the plasma were highest at 1 h and were below the limit of determination (< 20 ng/ml) at 5 h. The oral bioavailability of parent furazolidone administered in solution was 58%, compared with 28% in a feed mixture. 4. Concentrations of furazolidone and its metabolites were highest in the excretory tissues and lowest in the muscle after oral dosing. Parent furazolidone comprised 10% of the total 14C in the muscle at 8 h and was not detectable (< 1 ng/g) at 24 h; total 14C concentrations declined from 274 to 59 ng furazolidone equiv./g between 8 and 168 h. Non-extractable (bound) residues comprised 18% of total 14C in muscle at 8 h and 33% at 168h. 5. Renal excretion was the primary route of elimination of 14C residues and accounted for nearly 55% of the oral dose.

Liquid chromatographic determination of furazolidone in shrimp.

A liquid chromatographic (LC) method was developed for the quantitation of furazolidone residues in shrimp muscle. The shrimp homogenate (1.0 g) is extracted with acetonitrile, and the extract is taken to dryness. The residue is dissolved in acetonitrile, and the solution is passed through alumina and C18 cleanup columns. The eluate is taken to dryness and reconstituted in a suitable solvent for reversed-phase (C18) LC with UV detection at 365 nm. Recoveries of furazolidone from shrimp homogenates spiked from 5 to 80 ng/g ranged from 74.3 to 79.7%, and relative standard deviations (RSDs) were 5.0-8.9%. RSDs for incurred furazolidone quantitated at 5.9 and 9.2 ng/g were 6.6 and 7.6%, respectively.

Gas chromatographic determination of chloramphenicol residues in shrimp: interlaboratory study.

An interlaboratory study of a gas chromatographic method for determining chloramphenicol (CAP) residues in shrimp was conducted. An internal standard (Istd), the meta isomer of CAP, was added to the shrimp, and the treated shrimp were homogenized with ethyl acetate. The ethyl acetate extract was defatted with hexane, and the CAP was partitioned into ethyl acetate from an aqueous salt solution. The ethyl acetate was evaporated, and the dried residue was treated with Sylon, a trimethylsilyl derivatizing agent, to yield the trimethylsilyl derivative of CAP. A portion of the solution containing the derivative was injected into a gas chromatograph equipped with an electron capture detector. Levels of fortified and incurred CAP were calculated from the peak area ratio of standard CAP to Istd. Recoveries of CAP from tissue directly fortified at 5 ppb were 102% (within-laboratory relative standard deviation [RSDr] = 5.6%), 104% (RSDr = 5.5%), and 108% (RSDr = 6.3%) from Laboratories 1, 2, and 3, respectively. Incurred-CAP residues at 5 and 10 ppb levels were also determined, with the following results: Laboratory 1: composite A, 4.56 ppb (RSDr = 14.0%); composite B, 8.38 ppb (RSDr = 11.6%); Laboratory 2: composite A, 4.17 ppb (RSDr = 12.5%); composite B, 8.90 ppb (RSDr = 5.60%); Laboratory 3: composite A, 4.66 ppb (RSDr = 14.9%); composite B, 11.0 ppb (RSDr = 11.8%).

Simultaneous determination of nitrofurazone, nitrofurantoin, and furazolidone in channel catfish (Ictalurus punctatus) muscle tissue by liquid chromatography.

A liquid chromatographic (LC) method was developed for the simultaneous determination of nitrofurazone (NFZ), nitrofurantoin (NFT), and furazolidone (FZD) in catfish muscle tissue. The drugs were extracted from the tissue with acetonitrile, and the lipids were removed from the extract with hexane. The acetonitrile extract was evaporated by rotary evaporation, and the resultant drug residues were dissolved with LC mobile phase. The mixture was sonicated, centrifuged, and filtered. The drugs were determined by using LC with a C18 reversed-phase (ODS Hypersil) column, a mobile phase of acetonitrile-1% aqueous acetic acid (25 + 75), and a photodiode array ultraviolet detector at 375 nm. NFZ, NFT, and FZD were each determined in catfish tissue at 5 fortification levels (80, 40, 20, 10, and 5 ng drug/g tissue). Average recoveries of each of the 3 drugs at each level ranged from 70.7 to 101.5%, and relative standard deviations ranged from 2.2 to 18.6%. The limit of detection of each drug was approximately 1 ng drug/g tissue, and the limit of quantitation was 5 ng drug/g tissue. In the second part of the study, the method was used to determine nitrofuran residues incurred in catfish tissue. Live channel catfish were intravascularly doses (10 mg/kg body wt) with NFZ to generate drug-incurred fish muscle tissue. Incurred NFZ levels exceeded 400 ng drug/g tissue at 2 h after dosing but decreased rapidly to approximately 1 ng drug/g tissue by 8 h after dosing, as determined by this method.

Effects of dorsal aorta cannulation on the stress response of channel catfish (Ictalurus punctatus).

The stress response to dorsal aorta cannulation and serial blood sampling was examined in the channel catfish Ictalurus punctatus. Channel catfish cannulated and repetitively sampled once a day for 7 days did not exhibit a change in response to surgery or to the 24h sample regime as measured by plasma cortisol, glucose, and chloride. In fish that were either serially bled (0, 1, 3, 6, 9, 12, 24, and 48h) immediately after surgery or allowed to recover 6 days before being serially bled, overall plasma cortisol levels were higher than those of fish bled every 24h. Catfish serially bled immediately after surgery had significantly higher plasma glucose levels compared with catfish allowed to recover from surgery 6 days before serial sampling. Although channel catfish recover from cannulation surgery in 24h, a longer recovery period may be needed prior to serial sampling if the samples are taken more frequently than every 24h.

Pharmacokinetics and excretion of phenol red in the channel catfish.

1. Disposition of phenol red was examined in channel catfish (Ictalurus punctatus) after oral or intravascular (i.v.) dosing at 10 mg/kg body weight. 2. Phenol red was not detectable in plasma, urine, or bile after oral administration. 3. After i.v. dosing, plasma concentrations of phenol red were best described by a two-compartment pharmacokinetic model with distribution and elimination half-lives of 2.3 and 21 min, respectively. The apparent volume of distribution at steady state (Vss) was 225 ml/kg and total body clearance (Clb) was 658 ml/h per kg. Plasma protein binding was 19%. 4. Biliary excretion was the primary route of elimination of phenol red; in 24 h, 55% of the i.v. dose was excreted in bile compared with 24% in urine. No metabolites were detected in these fluids. 5. The use of anaesthesia during dosing had no effect on the quantitative excretion of phenol red by renal or biliary routes.

Disposition of 1-naphthol in the channel catfish (Ictalurus punctatus).

The pharmacokinetics, tissue distribution, and metabolism of 1-naphthol were examined in the channel catfish (Ictalurus punctatus). Catfish were administered [1-14C]1-naphthol intravascularly at 1, 5, or 25 mg/kg or orally at 1 mg/kg. Plasma data for 1-naphthol were fitted by a three-compartment pharmacokinetic model. There were dose-related changes in the area under the plasma concentration vs. time curve, apparent volume of distribution, and total body clearance after intravascular dosing. After oral dosing, peak plasma concentrations of 1-naphthol occurred at 1 hr; parent compound made up less than 15% of the total radioactivity, and the bioavailability was 32%. Plasma protein binding was 92% and was independent of concentration. 1-Naphthol and metabolites were rapidly eliminated from the tissues after oral dosing; less than 1% of the administered dose remained at 24 hr. Renal excretion was the primary route of elimination of total 14C. Approximately 60% of the oral dose was excreted in the urine within 48 hr. Parent 1-naphthol made up 1% of the urinary 14C. Major metabolites in the urine were sulfate and glucuronide conjugates, which composed 65 and 28% of the total 14C, respectively. Biliary excretion accounted for 7% of the oral dose. The glucuronide conjugate and an unidentified polar metabolite made up the majority of the biliary 14C. The high capacity of channel catfish for conjugative metabolism of 1-naphthol was demonstrated. The dose dependency of pharmacokinetic values could not be explained by saturable metabolism or plasma protein binding.

Tissue disposition and excretion of 14C-labelled aflatoxin B1 after oral administration in channel catfish.

The pharmacokinetics, tissue distribution and excretion of 14C-labelled aflatoxin B1 (AFB1) were examined after oral administration (250 micrograms/kg body weight) in channel catfish (Ictalurus punctatus). Plasma concentrations of parent AFB1 were best described by a one-compartment pharmacokinetic model, in which peak plasma concentration (503 ppb) occurred at 4.1 hr after dosing. The absorption and elimination half-lives were 1.5 and 3.7 hr, respectively. AFB1 was highly bound (95%) to plasma proteins. Concentrations of 14C (in AFB1 equivalents) measured in the tissues were highest at 4 hr, ranging from 596 ppb in the plasma to 40 ppb in the muscle. AFB1 residues were rapidly depleted; at 24 hr the concentrations in the plasma and muscle were 32 and less than 5 ppb, respectively. Concentrations in the bile exceeded 2000 ppb (at 24 hr), whereas the highest concentration in the urine was 51 ppb (4-6-hr collection interval). Renal and biliary excretion accounted for less than 5% of the administered dose, indicating incomplete absorption. Pharmacokinetic modelling and tissue data demonstrate a very low potential for the accumulation of AFB1 and its metabolites in the edible flesh of channel catfish through the consumption of AFB1-contaminated feed.

Bacillus stearothermophilus disk assay for determining ampicillin residues in fish muscle.

The Bacillus stearothermophilus disk assay for penicillin in milk (AOAC official method) was adapted for the determination of ampicillin in fish muscle. The method was evaluated in 2 species of cultured fish: channel catfish and striped bass. Recoveries of ampicillin ranged from 99 to 104% when muscle specimens from both species were spiked at concentrations of 0.025-1.00 micrograms/g. The lower limit of determination (LOD) was 0.025 micrograms/g. The assay was applied to monitor the elimination of ampicillin from the muscle of striped bass after intravascular administration (dosage of 10 mg/kg body weight). The mean concentrations in the muscle declined from 1.160 micrograms/g at 2 h to 0.063 micrograms/g at 18 h. The half-life of ampicillin in the muscle was 3.6 h. Ampicillin concentrations were below LOD at 24 h. No inhibitory activity was observed in the muscle of control fish.

Chlorpyrifos pharmacokinetics and metabolism following intravascular and dietary administration in channel catfish.

The pharmacokinetics and metabolism of a model organophosphorothioate, chlorpyrifos, was investigated in channel catfish (Ictalurus punctatus). Chlorpyrifos exhibited multicompartmental disposition after oral and intravascular administration, indicating slower distribution to peripheral storage tissues. The oral bioavailability of chlorpyrifos was 41%, substantially higher than in mammals. Muscle contained less than 5% of the oral dose, with an elimination half-life of about 3.3 days. Residues in the whole fish were greater than 95% chlorpyrifos, while excreta (bile and urine) primarily contained metabolites. The dephosphorylated metabolite, trichloropyridinol (TCP), was the major metabolite in the blood, while the glucuronide conjugate of TCP was the major metabolite in urine and bile. The toxic metabolite, chlorpyrifos oxon, was not detected in any samples of blood, tissues, or excreta. The metabolism of chlorpyrifos in catfish appeared similar to other species of fish and mammals. Extensive metabolism resulted in a low potential for chlorpyrifos to accumulate in catfish from dietary exposure.

Bioavailability, metabolism, and renal excretion of benzoic acid in the channel catfish (Ictalurus punctatus).

The pharmacokinetics and metabolism of the model compound benzoic acid were examined after intravascular (iv) and po administration at 10 mg/kg in the channel catfish (Ictalurus punctatus). A two-compartment pharmacokinetic model best described the plasma disposition of parent benzoic acid after iv dosing. The following pharmacokinetic values were estimated: elimination half-life, 5.9 hr; total body clearance, 61 ml/hr/kg; and volume of distribution (steady-state), 369 ml/kg. Plasma protein binding of [14C]benzoic acid was 18%. Benzoic acid was rapidly and extensively absorbed after po administration; absorption half-life was 0.8 hr and bioavailability was 95%. Renal excretion was the primary route of elimination of benzoic acid and metabolites. More than 80% of the iv-administered 14C was recovered in the urine in 24 hr. Unchanged benzoic acid comprised more than 90% of the urinary radiolabel. The major urinary metabolite was benzoyltaurine, which comprised 6-7% of the excreted 14C. Channel catfish were qualitatively similar to other teleost fishes in the formation of the taurine conjugate of benzoic acid. In contrast, the primary mammalian metabolite is the glycine conjugate, hippuric acid.