Twenty-five healthy women volunteers were selected to evaluate ovulation inhibition by a monophasic oral contraceptive preparation containing 75 micrograms gestodene and 30 micrograms ethinyl estradiol. Each subject participated for eight consecutive cycles, consisting of a pretreatment cycle, six treatment cycles, and a posttreatment cycle. During five explored cycles, serum LH, FSH, estradiol, and progesterone levels were measured daily on cycle days 8 through 17; in addition, progesterone was measured once, around cycle day 21. Pelvic ultrasounds were performed on cycle days 6, 8, 10, 12, 14, and 16. In the 18 volunteers completing the entire study, LH and FSH levels were strongly depressed, in equivalent degree, during the first, third, and sixth treated cycles. From treated cycle days 8 to 17, a significant decline of LH and FSH levels occurred, reaching values on the lower limit of detection. Luteal activity was not detected in any of the treated cycles. Follicular activity, as reflected by estradiol levels, was more strongly depressed during the first treated cycle (first contraceptive pill taken on day 1 of menstruation) than in the third and sixth treated cycles (the first pill taken after a seven-day pill-free interval). The excellent inhibition of follicular maturation was confirmed by ultrasonic assessment of the ovaries. Restoration of ovarian function during the first posttreatment cycle was excellent, showing a midcycle hormonal profile identical to that of the pretrial cycle.