The pharmacokinetics of tiaprofenic acid (TA) enantiomers was studied following oral administration of 300 mg of racemic TA in regular or sustained release (SR) capsules to four healthy male subjects, in cross-over fashion. The possibility of stereoselective release from either formulations was examined using the USP basket dissolution apparatus. In vitro dissolution rate from regular tablets was very rapid and comparable for S- and R-TA. While, the release from the SR capsules at pH 7.4 was slow and incomplete, with R-TA having 10.1 +/- 9.4% greater 24 h cumulative release than S-TA, the apparent stereoselectivity in dissolution disappeared at pH 8.0. After the regular formulation, TA was absorbed rapidly (Tmax, 1.5 h) and eliminated with a t1/2 of 2 h and superimposable plasma concentration-time curves. The SR formulation had a slower rate of absorption (Tmax, 4 h) and bioavailability comparable to that of the regular formulation. Similar to the examined regular release tablet, no significant stereoselectivity was observed in the plasma concentration and urinary excretion of the enantiomers after the SR formulation, suggesting that the dissolution test at pH 8.0 better reflects the in vivo conditions. Therefore, R-TA does not undergo significant chiral inversion in humans.