The chiral inversion kinetics of ibuprofen was evaluated after intraduodenal administration of racemic ibuprofen in conventional powder form and sustained-released granules compared with intravenous administration in rabbits. The AUC ratios of the S-(+) and R-(-) enantiomers remained almost constant values with time up to 2 h after administration of sustained-release formulation, while those after administration of the powder increased with time. R-(-) enantiomer to S-(+) enantiomer inversion ratios after intraduodenal administration of the powder form and the sustained-release form, and after intravenous injection were calculated to be 1.63, 1.94 and 1.19, respectively, indicating that pharmacological effects may depend on the absorption rate in rabbits.