have reviewed the literature on the ovarian hormones which we will again cite briefly from Johnstone1 and others.The reproductive cycle begins at the end of menstruation with the gradual maturation of the Graafian follicle which secretes a hormone called Estrin. Estrin is formed by all the developing follicles and probably the ovarian substance as well. It causes congestion of the genital tract and is a growth-promoting substance2. It seems to have some effect on the breasts as Kunde et al3 injected it into two normal and two castrated dogs who all showed enlargement of the breasts and one of whom had such copious lactation that she nursed two puppies for two weeks. The uterine wall from one showed hypertrophy of the endometrium.After ovulation takes place at the middle of the cycle of 28 days* a corpus luteum is formed which produces another hormone called Progestin. This internal secretion stimulates and governs the preparation of the uterus for nidation and gestation of the ovum, causing the thickening of and secretion in the glands of the endometrium, the so-called premenstrual phase of the menstrual cycle. It would be more correct to call it the pre-nidation phase in our opinion. If fertilization of the ovum does not occur the amount of progestin secreted is exhausted relatively quickly and menstruation occurs.It is known that if fertilization and nidation do occur the corpus luteum persists much longer than if menstruation comes on and also that the removal of a corpus luteum is followed in 2 or 3 days by a normal flow. Therefore it is assumed, but not proved, that the fertilized ovum must secrete some hormone which acts on the ovary to preserve the corpus luteum. Hisaw and Leonard2, in experiments on rabbits, appeared to show that estrin and progestin have to be used in succession to get the full progestational effect. Either alone does not achieve the desired result. This has already been applied clinically by Novak and others as we have said in previous reviews.Allen et al4 have tested various parts of the human ovary for its hormone content on castrated rats by injection of follicular fluid and by implantation of fresh corpora lutea, pieces of the follicle walls, and pieces of cortical stroma free of macroscopic follicles. Corpora lutea re-*Pemberton\p=m-\Member, Surgical Staff of the New England Deaconess and Palmer Memorial Hospitals. For record and address of author see "This Week's Issue," page 592. moved on the thirteenth to seventeenth days of the menstrual cycle seemed to have the largest content of hormone with a decrease in those of