A group of infertility patients were evaluated by an endometrial biopsy, timed with a basal body temperature chart, serum luteinizing hormone radioimmunassay to pinpoint ovulation, and daily serum progesterone values during a control and a treatment cycle. Progesterone in the suppository or intramuscular form and 17-hydroxyprogesterone caproate* were administered during the luteal phase to a group of volunteer patients with normal corpus luteum function to determine if these compounds would depress serum progesterone levels as do certain progestational agents. There was no apparent inhibition of corpus luteum function as no decrease in progesterone production occurred. Despite the additive effect of progesterone administration demonstrated by elevated serum levels, endometrial biopsies remained in phase when dated from the estimated day of ovulation.