Eleven normal female volunteers received human chorionic gonadotropin (HCG) intramuscularly to induce a pseudopregnant state. Twenty-five milligrams of prostaglandin F20(PGF20) was administered intravenously at selected times during the luteal phase to demonstrate a luteolytic effect, or a local effect upon the endometrium producing early vaginal bleeding. Careful comparison of hormone levels and clinical details in these cycles showed that a) 1000 IU HCG daily does not consistently prolong the luteal phase; b) 2500 IU HCG daily does significantly prolong the luteal phase;c) HCG administration significantly increases progesterone and estradiol output in the treated luteal phase;d) prostaglandin administration at selected times during the luteal phase does not consistently decrease estradiol or progesterone output from the corpus luteum or induce early vaginal bleeding;e) prostaglandin F20, a known stimulant of uterine contractility, probably causes vaginal bleeding by mechanical means, as evidenced by show of menses in patients with progesterone levels elevated above normal;f) although variations in progesterone and estradiol output occurred during the hours of prostaglandin infusion, no consistent change was observed except for a compensatory rebound in progesterone output;g) prostaglandin F20, in the dose utilized, is not a luteolytic agent.