The term "progestin" was applied by Allen1 and by Corner to the hormone of the corpus luteum, which maintains in a normal state of pregnancy rabbits castrated the day after mating. In experimental work, progestin has been found to produce most of the effects already known to be associated with the functioning corpus luteum. These functions are characteristic proliferation of the endometrium for the nourishment and implantation of ova, growth of the decidua, maintenance of pregnancy and inhibition of ovulation. Excess of progestin, by raising the threshold of uterine muscle by stimulation, has been shown to inhibit the delivery mechanism, with consequent death of pregnancy in utero. In addition, we have found2 that progestin, through stimulation of the suprarenal glands, may have other functions, both metabolic and vasomotor. In the course of this paper another activity of progestin, in relation to the excretion of the ovarian follicle hormone,