Abstract Pre- and postovulatory states of ovaries were induced by the injection of PMSG and PMSG + hCG treatments, respectively, to immature rats. The concentration of ovarian estradiol measured by radioimmunoassay decreased significantly following hCG treatment to PMSG-pretreated rats. Subcutaneous administration of testosterone in soybean oil-glycerol mixture (9:1, v/v) restored the decreased concentration of the ovarian estradiol markedly in the PMSG + hCG treated rats, but not in the group treated with PMSG alone or in the control group treated with no gonadotropin. On the other hand, 5α-dihydrotestosterone showed no increase in the ovarian estradiol of any group. When an ethanol solution of testosterone was administered s.c. to the PMSG + hCG treated rats, the ovarian estradiol level was maximally enhanced from 0.5 to 1.0 h after the injection. On the other hand, 5α-dihydrotestosterone, dehydroepiandrosterone and 17α-hydroxyprogesterone in ethanol showed no effect l h after the injection. These results indicate that the drastic decrease in ovarian estradiol production due to the hCG administration is caused by an acute decrease in the supply of aromatizable androgens to ovarian aromatase.