1. The stereoselective disposition of ofloxacin (OFLX) was studied in rats, dogs and monkeys after oral administration of racemic OFLX. 2. In rats serum concentrations of (R)-(+)-OFLX were much greater than those of (S)-(-)-OFLX, which is the active form of OFLX. In monkeys, by contrast, serum concentrations of (S)-(-)-OFLX predominated over (R)-(+)-OFLX levels. In dogs there were no differences in AUC or Cmax between the enantiomers. Thus, there exists a species-related difference in the stereoselective disposition of OFLX. 3. In rats the stereoselective differences were mainly due to stereoselective glucuronidation; OFLX is hardly metabolized in dogs, monkeys and humans. 4. In monkeys the AUC of (S)-(-)-OFLX was increased by co-administration of the (R)-(+)-form, indicating that the stereoselectivity of OFLX disposition in monkeys may be caused by competition between the enantiomers for renal excretion, especially for renal tubular secretion.