introduction Prevention of excessive follicular growth and of follicular bleeding in the intrasplenic ovarian graft, and enhancement of luteinization, have been produced in the castrated guinea pig to which oestrogen has been administered (Lipschutz a. oth. 1948; Barahona et al. 1950). These results have been interpreted, in accordance with the prevailing concept of the ovarian dynamics, as due to the action of oestrogen on the gonadotrophic function of the hypophysis. But it must be borne in mind that oestrogen has been found to cause ovarian changes also in hypophysectomized animals. Indeed, these actions of the hormone are not always coincident with those taking place in the intact animal; stimulation instead of inhibition of follicular growth in hypophysectomized rats has been reported by various authorities (Pencharz 1940; Williams 1940, 1944; Simpson et al 1941). However, stimulation of follicular growth has been elicited by oestrogen also in intact immature rats (Gaarenstroom 1942; Gaarenstroom and De Jongh 1943).