We studied the effects of shakuyaku-kanzo-toh (a Chinese herbal medicine) and its components on testosterone production by rat gonads. We used paeoniflorin as a main component of shakuyaku (paeoniae radix), glycyrrhizin as a main component of kanzo (glycyrrhizae radix) and glycyrrhetinic acid as a main metabolite of glycyrrhizin. Oral administration of shakuyaku-kanzo-toh, glycyrrhizin, and glycyrrhetinic acid decreased in vitro basal testosterone production in Leydig cells by LH stimulation. Glycyrrhizin and glycyrrhetinic acid caused a significant decrease in testosterone production with an accumulation of 17 alpha-hydroxyprogesterone when incubated with isolated Leydig cells, while paeoniflorin showed no such effect. The inhibitory effect of glycyrrhetinic acid was far more potent than that of glycyrrhizin, causing about 90% inhibition at 10 micrograms/ml. Glycyrrhizin and glycyrrhetinic acid did not change the cyclic AMP or progesterone level in the Leydig cells. When 14C-labeled androstenedione was incubated with microsomal fraction of testicular or ovarian tissue, glycyrrhizin and glycyrrhetinic acid inhibited the conversion of androstenedione to testosterone, indicating that these compounds inhibit the activity of 17 beta-hydroxysteroid dehydrogenase (EC. 1.1.1.64). The ED50 of glycyrrhetinic acid was about 4 microM.