A combination of 1.0 mg dl-norgestrel and 0.1 mg ethinylestradiol (EE) was administered orally at 36 hours after the detection of the luteinizing hormone peak and again at 48 hours in 12 healthy volunteers with normal menstrual cycles. The effects on ovarian function were studied by comparing the daily serum levels of progesterone (P), 17 alpha-hydroxyprogesterone, and estradiol (E2) in control (placebo) and treatment cycles. Five subjects showed no significant change in the levels of these steroids but had a shortened luteal phase. The treatment significantly decreased both P and E2 levels in three subjects, while two subjects showed diminished E2 levels only. The remaining two subjects had lower P levels and fluctuating E2 patterns. Endometrial biopsies from both study cycles indicated asynchronous development of the epithelial and stromal components in the treatment cycle. These findings (abnormal luteal phase steroid levels and duration and outphased endometrial development) indicate that corpus luteum function was variously affected by the action of norgestrel-EE treatment.