The pharmacokinetics of oxytetracycline (OTC) were investigated in rainbow trout in freshwater at 10.5 degreesC. A group of 70 fish received OTC, an intravascular injection (i.v.) of 5 mg/kg body weight. Another group of 105 fish were fed medicated feed containing a dose of 100 mg/kg body weight. OTC. plasma concentrations were determined by high performance liquid chromatography. The pharmacokinetic parameters were described by a two-compartment open model giving distribution half-lives of 0.907 h and 16.058 h, and elimination half-lives of 33.5 h and 27 h for Lv. and oral administration of OTC, respectively. After i.v. administration of OTC, other pharmacokinetic parameters estimated were the area under the curve (AUC: 145.078 +/- 4.45 mug.ml(-1).h(-1)) and mean residence time (MRT: 40.604 +/- 1.150 h). Following oral administration, AUC was 34.46 +/- 1.189 mug.ml(-1).h(-1); MRT was 59.272 +/- 1.541 h; C-max was 0.577 mug/ml and T-max was 36 h. The oral bioavailability (F) of OTC in rainbow trout was found to be only 1.187%.