A single injection of 62.5 µg reserpine inhibits ovulation induced in intact immature rats by administration of pregnant mare’s serum (PMS) and human chorionic gonadotrophin (HCG) when reserpine is given ½ to 6½ h before HCG. Increases or decreases in the amount of HCG are not reflected by similar changes in the ovulatory response of reserpinized rats, indicating that reserpine’s action is not due to an antagonism of HCG. The monoamine oxidase inhibitor (MAOI), pargyline, partially reverses the inhibitory effect of reserpine, although it has no effect on ovulation per se. Previous experiments with reserpine suggested that its inhibitory effect on ovulation was due to its ability to block luteinizing hormone (LH) release. The present experiments circumvent the necessity for consideration of LH release, since HCG is given exogenously in amounts adequate to induce ovulation. It is clearly evident from our results that reserpine exerts a direct effect on the ovary in addition to its effect at the hypothalamo-hypophyseal level.