The effect of absorption rate on the pharmacokinetics of ibuprofen enantiomers was investigated in 12 healthy Han Chinese male volunteers following oral administration of immediate-release (IR) and sustained-release (SR) preparations containing racemic ibuprofen (rac-ibuprofen). The area under the curve of the plasma concentration-time curve (AUC; (mean+/-s.d.) values for rac-ibuprofen were 192.90+/-43.47 for the SR preparation and 195.90+/-31.69 microg h mL(-1) for the IR preparation. AUC values for the enantiomers after administration of the SR formulation were 55.38+/-17.79 and 92.51+/-30.68 microg h mL(-1) for R- and S-ibuprofen, respectively, and were 65.94+/-20.06 and 100.81+/-32.28 microg h mL(-1) for R- and S-ibuprofen after administration of the IR preparation. These values did not differ significantly. C(max) values were significantly decreased with the SR preparation: 25.11+/-5.71, 12.24+/-3.79 and 12.38+/-3.55 microg h mL(-1) for rac-, R-, and S-ibuprofen, respectively, after administration of the SR preparation, vs 46.21+/-8.20, 20.82+/-5.90 and 23.46+/-7.30 microg h mL(-1) for rac-, R-, and S-ibuprofen, respectively, after administration of the IR preparation. Mean residence time was significantly increased: 7.01+/-1.29, 5.52+/-1.25 and 7.04+/-1.30 h for rac-, R-, and S-ibuprofen, respectively, after administration of the SR preparation vs 4.34+/-0.89, 3.43+/-0.64 and 4.51+/-0.79 h for rac-, R-, and S-ibuprofen, respectively, after administration of the IR preparation. AUC values for S-ibuprofen were significantly larger than those for R-ibuprofen in both preparations, indicating unidirectional chiral inversion. The S/R ratio of serum concentrations of enantiomers was 1.78-fold higher at 6 h after administration of the SR preparation compared with the IR preparation (P<0.01). These results indicate that ibuprofen undergoes pre-systemic chiral inversion in parallel with a systemic process and that the clinical effects of rac-ibuprofen in humans depend on the absorption rate.