1. Branchial and urinary elimination of benzocaine residues was evaluated in adult rainbow trout, Oncorhynchus mykiss, given a single dorsal aortic dose of 14C-benzocaine hydrochloride. 2. Branchial elimination of benzocaine residues was rapid and accounted for 59.2% of the dose during the first 3 h after dosing. Renal elimination of radioactivity was considerably slower; the kidney excreted 2.7% dose within 3 h and 9.0% within 24 h. Gallbladder bile contained 2.0% dose 24 h after injection. 3. Of the radioactivity in radiochromatograms from water taken 3 min after injection, 87.3% was benzocaine and 12.7% was N-acetylated benzocaine. After 60 min, 32.7% was benzocaine and 67.3% was N-acetylated benzocaine. 4. Of the radioactivity in radiochromatograms from urine taken 1 h after dosing, 7.6% was para-aminobenzoic acid, 59.7% was N-acetylated para-aminobenzoic acid, 19.5% was benzocaine, and 8.0% was N-acetylated benzocaine. The proportion of the radioactivity in urine changed with time so that by 20 h, 1.0% was para-aminobenzoic acid and 96.6% was N-acetylated para-aminobenzoic acid. 5. Benzocaine and a more hydrophobic metabolite, N-acetylated benzocaine, were eliminated primarily through the gills; renal and biliary pathways were less significant elimination routes for benzocaine residues.
