To examine the possible effects of Tokishakuyakusan (TS) on steroidogenesis by preovulatory follicles at the cell level, the expressed granulosa cells and remaining portion of follicles from pregnant mare's serum gonadotropin (PMS)-treated immature rats were incubated in vitro with increasing concentrations of TS for 3 h. TS significantly stimulated progesterone and estradiol-17 b production, with a predominant stimulation of progesterone, by the expressed granulosa cells, while testosterone production was not stimulated. In the remaining portion of the follicle, TS also significantly stimulated progesterone, testosterone and estradiol-17 b production. Similar to the effect produced by granulosa cells, the stimulatory effect of TS was stronger on progesterone than on testosterone and estradiol-17 b production. These results suggest that TS has a potent, direct stimulatory effect on steroidogenesis, especially progesterone production, by constituent tissue compartments of rat preovulatory follicles in vitro.