In earlier work1it has been demonstrated that lipoid extracts of the contents of ovarian follicles quite completely substitute for certain phases of the internal secretion of the ovaries. This was done by injecting extracts of material from pig ovaries into rats and mice from which the ovaries had previously been removed. The action of the follicular hormone includes the production of striking growth in the female genital organs. The rapidity of this growth in the test animals is astonishing. An entirely new epithelial wall is produced in the vagina in forty-eight hours. This involves the addition of more than twelve layers of new cells (Fig. 2, in whichA, the control animal, is to be compared with B and C, injected animals). In the uterus a corresponding growth occurs and the secretion of the glands, which is retained in these animals by cervical constriction, greatly distends the uterus