S(+)-, R(-)- or racemic ibuprofen was administered orally to volunteers in doses of 150 mg, 300 mg and 500 mg pure S(+)-, 300 mg pure R(-)- and 600 mg racemic ibuprofen. The pharmacokinetic parameters in humans showed that S(+)-ibuprofen was not inverted to R(-)-ibuprofen, whereas R(-)-ibuprofen was inverted to S(+)-ibuprofen to a variable degree. S(+)-ibuprofen and R(-)-ibuprofen given alone more rapidly reached significantly higher maximal plasma concentrations than after the same doses of the racemic compound. The elimination half-lives and clearance values for all three forms of ibuprofen were comparable. The mean residence time of S(+)-ibuprofen after R(-)- and racemic ibuprofen was significantly longer than after administration of the pure S(+)-enantiomer. Judged by the AUC, the bioavailability of S(+)-ibuprofen was independent of the dose within the range tested. Administration of S(+)-ibuprofen to 6 rheumatic patients showed that the pharmacokinetic behaviour of S(+)-ibuprofen in patients was similar to that found in volunteers. S(+)-ibuprofen proved to be an effective analgesic antirheumatic drug in the dose range 1 to 1.5 g/day.