There are indications that, at least in blinded hamsters or in hamsters exposed to short daily photoperiods, the pineal gland may secrete a gonadotrophin-inhibiting substance (Hoffman & Reiter, 1965a; Reiter, Hoffman & Hester, 1966a, b). The ovarian and testicular atrophy which ensues after hamsters are blinded is prevented by surgical removal of the pineal gland. Although several workers have speculated on the mechanism by which pineal substances produce the inhibitory effect, the problem remains unsolved (Moszkowska, 1965; Reiter & Hester, 1966). The present results show that the inhibitory substance of pineal origin neither inhibits completely the action of exogenous gonadotrophins on the ovary nor the action of oestrogen on the uterus. Thirty young adult female hamsters (Mesocricetus auratus) were divided into 4 groups: (1) untreated controls; (2) animals subjected to bilateral optic enucleation (blinding); (3) hamsters that were both enucleated and pinealectomized; (4) blinded hamsters that were injected