This study has examined the stereoselective disposition of the enantiomers of ibuprofen in four healthy male subjects following separate administration of racemic ibuprofen (800 mg) and of each enantiomer (400 mg). A mean of 63 +/- 6% of an administered dose of R(-) ibuprofen was stereospecifically inverted to the S(+) enantiomer. There were no measurable inversion of the S(+) to R(-) ibuprofen. The kinetics of the individual enantiomers were altered by concurrent administration of the respective optical antipode. It is likely that this change reflects an interaction between the enantiomers at plasma protein binding sites. It was found that formation of ester glucuronide conjugates stereoselectively favoured the S(+) enantiomer. The data have demonstrated that the pharmacokinetics of ibuprofen and other alpha-methylarylacetic acids cannot be interpreted adequately without studying the pharmacokinetics of the individual enantiomers.