The pharmacokinetics of intravenously and orally administered enrofloxacin was determined in fingerling rainbow trout (Oncorhynchus mykiss). Doses of 5 or 10 mg enrofloxacin/kg body weight were administered intravenously to 26 fish for each dose and blood was sampled over a 60-h period at 15 degrees C. Two groups of fish were treated orally with 5, 10, or 50 mg/kg (80 fish/dose at each temperature) and held at 15 degrees C or 10 degrees C during the 60-h sampling period. Following intravenous administration, the serum concentration-time data of enrofloxacin in rainbow trout were best described by a two-compartment open model for both doses of 5 and 10 mg enrofloxacin/kg. The hybrid rate constants alpha and beta did not differ between doses. The distributional phase was rapid with a half-life of 6-7 min for both doses. Overall half-lives of elimination were 24.4 h (95% CI = 20.2-30.8) and 30.4 h (24.2-41.0), respectively, for the 5- and 10-mg/kg doses. A large Vd(area) was observed following dosing of either 5 or 10 mg enrofloxacin/kg,: 3.22 and 2.56 l/kg, respectively. Whole body clearance for 5 mg/kg was 92 ml/h.kg and 58 ml/h.kg at the 10-mg/kg dose. Following oral administration, the serum concentration-time data for enrofloxacin were best described as a one-compartment open model with first-order absorption and elimination. Apparent Ka over all doses at 10 degrees C averaged 62% less than apparent Ka at 15 degrees C. Estimates of the apparent t(1/2)e over both temperatures ranged from 29.5 h (18.4-73.4) to 56.3 h (38.3-106.6). Bioavailability averaged 42% over all doses at 15 degrees C and was decreased to an average of 25% at 10 degrees C. Peak serum concentrations appeared between 6 and 8 h following dosing. A dose of 5 mg/kg/day was estimated to provide average steady-state serum concentrations at 10 degrees C that are approximately 4.5 times the highest reported MIC values for Streptococcus spp., the fish pathogen least sensitive to enrofloxacin. Owing to the long apparent half-life of elimination of enrofloxacin in fingerling trout, it would take approximately 5 to 9 days to achieve these predicted steady-state serum concentrations; this estimate is important when considering the duration of therapy in clinical trials.