The pharmacokinetics of a single 400 mg oral dose of fleroxacin and pefloxacin were evaluated in ten healthy male volunteers in a randomized cross-over study. There were no significant differences in Tmax (0.9 vs. 1.3 h) and in plasma elimination half-life (11.9 vs. 10.8 h) between fleroxacin and pefloxacin. Cmax and AUC of fleroxacin were statistically significantly greater (P less than 0.05) compared to pefloxacin (Cmax: 5.62 vs. 4.09 mg/l, AUC0-48: 65.9 vs. 48.7 mg/l.h, AUC0-infinity: 70.7 vs. 51.5 mg/l.h). Renal clearances of fleroxacin and pefloxacin were 51.8 and 11.7 ml/min respectively. The 48-h urine recovery was 48.6% for fleroxacin, 8.6% for pefloxacin, 7.1% and 17.4% for the N-demethyl metabolites, and 3.8% and 16.6% for the N-oxide metabolites of fleroxacin and pefloxacin respectively. Urinary concentrations of both the microbiologically active parent drug and the N-demethyl metabolite of fleroxacin were, at all intervals up to 48 h post dose, two to three times higher than those of pefloxacin.