Pharmacokinetic behaviour of enrofloxacin was studied in 6 horses after intravenous (i.v.) or intramuscular (i.m.) administration of enrofloxacin (5 mg/kg bwt). Concentration of enrofloxacin and ciproflaxin were measured by high performance liquid chromatography in serum. Antimicrobial activity of the samples was determined with an agar-diffusion technique. Reactions at the site of i.m. injection were monitored clinically and by determination of serum creatine kinase (CK) activity. After i.v. administration, elimination half-life of enrofloxacin was 4.4 h and volume of distribution was 2.3 1/kg bwt. Enrofloxacin was rapidly metabolised to ciprofloxacin. The half-life of ciprofloxacin parallelled that of the parent drug, its concentration in serum reached 20-35% of that of the parent drug. After i.m. administration, elimination half-life of enrofloxacin was longer (9.9 h) than after i.v. administration. Mean absorption time of enrofloxacin was also long (9.9 h). No statistically significant differences were found when half-life and mean residence time of antimicrobial activity were compared with those of enrofloxacin and ciprofloxacin from chemically analysed data. Intramuscular injection of enrofloxacin was found to be very irritating. After i.m. administration, CK activity in serum, compared with pre-injection levels, increased over 10-fold. CK activity also stayed high during the 32 h follow-up period. Clinical reactions, such as swelling or tenderness at the i.m. injection sites, were observed in 2 horses.