The pineal gland exerts an inhibitory influence on the gonads and although the exact mechanism is unknown, Wurtman, Altschule & Holmgren (1959) have postulated that melatonin may be the active pineal substance. Recent evidence indicates that one site of action is at the hypothalamo—hypophysial system since implantation of melatonin, 5-hydroxytryptophol, or fragments of pineal tissue, into the median eminence is followed by a significant reduction of pituitary and plasma luteinizing hormone (LH) (Fraschini, Mess & Martini, 1968). Sorrentino (1968) has shown that daily injections of melatonin decrease serum follicle-stimulating hormone (FSH) levels in the intact female rat. Markedly depressed serum FSH levels have been observed in the castrated female rat after a single injection of melatonin (B. Benson, M. K. Vaughan & J. T. Norris, unpublished results). Daily administration of a pineal extract (Reiss, Davis, Sideman, Mauer & Plichta, 1963) or of melatonin (Sorrentino, 1968) blocked compensatory ovarian hypertrophy after