An open-label, single-center, noncomparative study was conducted to determine the effects of a monophasic oral contraceptive containing 100 microg levonorgestrel and 20 microg ethinyl estradiol on ovarian activity. The subjects were 26 healthy women 20 to 35 years of age who had normal ovulatory cycles and were not at risk for becoming pregnant. For 3 treatment cycles, they took 1 tablet of active drug daily for 21 days followed by placebo tablets for 7 days. Follicle diameters and serum progesterone and 17beta-estradiol levels were measured before, during, and after treatment. In 2 (2.7%) of 73 cycles, luteinized unruptured follicles were present and in another 2 (2.7%) cycles, ovulation was confirmed by the disappearance of the enlarged follicle. Ovarian activity, as reflected by mean serum progesterone levels, was restored after treatment. The results of this study are in agreement with those of other studies that showed suppression of ovarian activity in women treated with a monophasic oral contraceptive containing 100 microg levonorgestrel and 20 microg ethinyl estradiol. These results indicate that low-dose 100 microg levonorgestrel and 20 microg ethinyl estradiol given for 21 days is effective in suppressing ovarian activity and they confirm the contraceptive efficacy observed in clinical trials (Pearl index of 0.8).