A study was made to observe the influence of pinealectomy on the excretion of a gonadotropin inhibitor in the urine of male rats. Ovulation induced with PMS and HCG was not inhibited in mice which received a heated extract prepared from the urine of pinealectomized rats, whereas ovulation was inhibited in mice given a heated extract from the urine of intact and of sham-operated rats. Ovulation was also partially inhibited in mice receiving a heated extract from the urine of hypophysectomized rats. These observations suggest that the pineal may be the secreting organ of a gonadotropin inhibitor and that the pituitary may be involved in the secretion of the inhibitor in rats. (Endocrinology88: 816, 1971)