We investigated the pharmacokinetics of prednisolone (PSL) in six healthy men, with or without glycyrrhizin (GL), to confirm whether GL influences the metabolism of PSL in humans. Each subject received an intravenous administration of 0.096 mg/kg of prednisolone hemisuccinate (PSL-HS, equivalent to 0.075 mg/kg of PSL), with or without 200 mg of GL. Blood samples were taken from a peripheral vein at 5, 10, 15, 30 and 45 min, and 1, 1.5, 2, 3, 4, 6, 8, 10, 12 and 24 h after PSL-HS infusion. The concentration of total PSL in the plasma was analyzed by high-performance liquid chromatography, and the free PSL was measured by an isocolloidosmolar equilibrium dialysis method. The pharmacokinetic parameters of PSL were determined, using noncompartmental analysis. GL was found to increase significantly the concentration of total PSL at 6, 8 h, and of free PSL at 4, 6, and 8 h after PSL-HS infusion. GL was also found to modify the pharmacokinetics of PSL. After the administration of GL, the area under the curve (AUC) increased, total plasma clearance (CL) decreased, and the mean residence time (MRT) was prolonged. However, only those of AUC, CL, and MRT of free PSL were significantly different. The volume of distribution at a steady-state (Vdss) of both total and free PSL showed no evident change. This suggests that GL increases the plasma PSL concentrations by inhibiting the metabolism of PSL and that it potentiates pharmacological effects of PSL.