A high-pressure liquid chromatographic method for the analysis of rosoxacin and its pyridyl N-oxide metabolite in plasma and urine extracts is described. A statistical evaluation of the assay data has shown acceptable accuracy and precision for 0.5 to 25 microgram of rosoxacin or the metabolite per ml of plasma and for 2.5 to 60 microgram/ml of either compound in urine. The minimum quantifiable level for rosoxacin was 0.13 microgram/ml in plasma and 0.64 microgram/ml in urine; for the metabolite in plasma and urine, the corresponding values were 0.21 and 0.60 microgram/ml, respectively. The method was applied to plasma and urine from three dogs medicated orally with 5 mg/kg of rosoxacin. The pharmacokinetic parameters calculated for rosoxacin were: plasma halflife, 1.9 h; plasma clearance, 65 ml/min; volume of distribution, 11.31. The average total urinary excretion of rosoxacin as free and conjugated rosoxacin and its free N-oxide was 7.7 +/- 0.2% over the 48-h collection period.