Both an extract of sow''s corpora lutea and progesterone, prepared from stigmasterol, inhibited ovulation after mating in the rabbit. This inhibition did not extend more than 48 hrs. after the last injection of the hormone. Many of the post-partum (oestrous) rabbits were thrown out of heat by the injections and would not accept the buck. In rabbits receiving a corpus luteum preparation in doses equal to or greater than amounts adequate to sustain pregnancy, the ovarian follicles will ovulate in response to a minimal ovulating dose of a gonadotropic preparation. The action of progesterone, as disclosed in these exps., is sufficient to explain the inhibition of ovulation during pregnancy and pseudopregnancy in the rabbit. This inhibition is not exercised directly upon the ovarian follicles, but at a point more central in the ovulation provoking mechanism.