Enantiomeric compositions of the major urinary metabolites of ibuprofen [(RS)-2-(4-isobutylphenyl)propionic acid]were characterized after oral administration of the racemic mixture and oral administration of the individual enantiomers to normal human volunteers. Resolution of the diastereomeric amides, formed by reaction of the urinary metabolites with (S)-(-)-alpha-methylbenzylamine, was achieved by GLC. Only the (R)-(-)-enantiomer of the intact drug was inverted to its optical antipode, (S)-(+), in humans. However, both (S)-(+)- and (R)-(-)-enantiomers of the intact drug were transformed independently in vivo to the major metabolites, i.e., 2,4'-(2-hydroxy-2-methylpropyl)phenylpropionic acid and 2,4'-(2-carboxypropyl)phenylpropionic acid. In vivo metabolism of ibuprofen to its carboxy metabolite was not stereoselective.