Reproductive involution produced by surgical removal of the olfactory bulbs and of the eyes from adult female rats was prevented by pinealectomy, superior cervical ganglionectomy or by bilateral transection of the medial forebrain bundle within the lateral hypothalamus. The activity of the melatonin-forming enzyme, hydroxyindole-O-methyltransferase, was diminished after sympathetic denervation of the pineal gland; this decreased activity of the pineal probably explains its failure to induce gonadal regression in blinded, anosmic rats. Interruption of the medial forebrain bundle may have prevented involution of the ovaries and associated sexual structures by at least two different means. The transections may have interrupted the inferior accessory optic tract, which transmits information about photic stimuli to the pineal gland or, alternately, they may have negated the pineal’s effect by interrupting afferents to the hypothalamus from cell bodies within the mesencephalon, which are responsive to pineal secretory products.