The tolerance and pharmacokinetics of fleroxacin were studied in healthy male adult volunteers. The peak serum concentrations of unchanged fleroxacin were about 1.5, 3 and 5 mg/l at 1-2 h after single oral doses of 100, 200 and 400 mg, respectively. The apparent serum elimination half-life was about 10 h, independent of the dose. Fleroxacin, demethyl fleroxacin and fleroxacin N-oxide excreted in urine over 3 days accounted for about 75%, 5% and 5%, respectively, of the doses. The urine concentrations of unchanged drug were dose-related; the mean concentrations, sustained over 24 h, were about 50, 100 and 150 mg/l after 100, 200 and 400 mg doses, respectively. Food intake did not significantly influence the serum concentration and urinary excretion. Steady state serum concentrations were achieved from day 3 onwards by repeated doses of twice-a-day dosage regimen and were 2-4 and 5-9 mg/l after 200 and 400 mg bid, respectively. The mean concentrations of unchanged drug in urine were about 200 and 300 mg/l at the respective dosages. The pattern of urinary metabolites was not changed by repeated doses and 90% of repeat doses was recovered in urine, including metabolites. The serum protein binding of fleroxacin was 32%. The saliva concentration was 40% of the total serum concentration or 60% of the free serum concentration. The faecal recovery over 3 days was 3% of the dose following a single 200 mg dose after a meal. The unchanged drug concentrations in faeces during 400 mg repeated dosing were 100-150 mg/kg. No severe dose-related side-effects were observed during the study.