OBJECTIVETo investigate the pharmacokinetics of enrofloxacin in adult horses.DESIGN2-dose oral and i.v. cross-over trial followed by multiple oral doses.ANIMALS8 clinically normal adult horses.PROCEDUREEnrofloxacin was administered at dosages of 2.5 mg/kg of body weight to 4 horses and 5.0 mg/kg to 4 other horses. Each dose was given by the intragastric and i.v. routes, using a cross-over design. After the first intragastric dose, 5 additional doses were administered at 12-hour intervals. Enrofloxacin concentrations were measured in serum, synovial fluid, peritoneal fluid, urine, CSF, and endometrial tissues.RESULTSDisposition of enrofloxacin after i.v. administration conformed to a 2-compartment model with an elimination half-life of 5.95 and 6.09 hours for the low and high dose, respectively. After the first intragastric administration, time to peak concentration was 1.0 +/- 0.35 and 1.25 +/- 0.43 hours, and the bioavailability was 57.39 +/- 8/45 and 62.52 +/- 19.65% for the low and high dose, respectively. After multiple intragastric administration, peak serum concentration (at 72 to 96 hours) was 2.62 +/- 0.61 micrograms/ml for the low dose and 5.97 +/- 1.56 micrograms/ml for the high dose. After multiple intragastric doses, synovial fluid, urine, and endometrial tissue concentrations exceeded serum concentrations. Peritoneal fluid and CSF concentrations were lower than serum concentrations.CONCLUSIONSComputer modeling of the pharmacokinetic variables suggested that a single daily i.v. administered dose of 5.5 mg/kg, or orally administered doses of 7.5 mg/kg every 24 hours or 4.0 mg/kg every 12 hours, would be effective in horses. Additional studies are necessary to confirm the efficacy and safety of these dosages in a clinical setting.