Experimental studies of pregnancy in laboratory animals have shown that if the placentae are left attached to the uterus while all the foetuses are removed, the functional activity and histological structure of the corpora lutea are maintained. Klein [1933] performed the operation on the rabbit and repeated it subsequently on the rat [Klein, 1935a, b] and on the golden hamster [Klein, 1938]. Similar experiments in the rat were carried out by Selye, Collip & Thomson [1935 b] and certain results of placental retention in the mouse were described by Newton [1935]. Newton & Lits [1938] showed that most of these results necessitated the simultaneous presence of the ovaries. Newton & Beck [1939] have since shown that the pituitary gland can be removed without materially affecting the results, and consequently a relationship in which the pituitary is not concerned exists between the placentae and the ovaries. Hypophysectomy, unlike ovariectomy, commonly