The occurrence of a characteristic cell in the anterior lobe of the hypophysis of spayed animals has been recognized since the pioneer work of Fichera in 1904, and 2 important biological phenomena have recently been demonstrated accompanying this histological change. It has been found that implants of the anterior lobe of castrated rats are much more potent in inducing precocious maturity than those from normal animals,1, 2 possibly due to a storage of the hormone by the castration cells.2 It has also been found that the blood3 and urine4 of women after bilateral oophorectomy contains large amounts of an ovary-stimulating substance as determined by the Ascheim-Zondek test.Since it has also been suggested that estrin (ovarian follicular hormone) has the power of inhibiting the ovary-stimulating action of the anterior hypophysis (Siegmund5; Mahnert6; Dahlberg and Akesson7; Meyer, Leonard, Hisaw and Martin8), it seemed of importance to determine if the constant administration of large amounts of estrin to sp...