Homoplastic anterior pituitaries were transplanted beneath the renal capsule of adult male, Syrian golden hamsters and the effects studied on the gonadal and accessory organ atrophy following bilateral optic enucleation or placement in reduced (L/D = 10/14 h) photoperiods. In all cases, control hamsters, bearing kidney fragments transplanted beneath their renal capsules, showed gonadal atrophy and reduced blood levels of prolactin and LH at 8–10 weeks following light deprivation. On the other hand, hamsters, bearing three or four homoplastic pituitary grafts, showed complete maintenance of gonadal and accessory weights as well as normal or supranormal serum prolactin levels. Prolactin, but not LH, levels were maintained in hypophysectomized male hamsters bearing pituitary homografts. These results point out the significant role played by prolactin, and possibly that of pineal prolactin-regulating factors, in pineal-mediated gonadal atrophy.