Twenty-four healthy female volunteers with normal ovulatory cycles, aged between 20 and 34 years (27.5 +/- 4.3), were included in a single-center, non-comparative study to investigate the effect on inhibition of ovulation of an oral contraceptive containing 20 micrograms ethinylestradiol in combination with 100 micrograms levonorgestrel. At baseline, during three treatment cycles and post-treatment, ultrasonography was used to examine the ovaries, to measure follicular size, and to measure the thickness of the endometrium. Serum levels of LH, FSH, estradiol, progesterone, total testosterone, free testosterone, SHBG, and CBG were also measured. Compared with treatment cycle 1, an increase in residual ovarian activity (follicle grades 4-5) was observed in cycles 2 and 3. Mean levels of LH, FSH, 17 beta-estradiol and progesterone remained suppressed during treatment. No escape ovulation was observed during the treatment phase of the study and there were no pregnancies. Ovulation was noted to return rapidly in the posttreatment cycle. Subjective and objective tolerance of the present regimen was noted to be good. Results indicate that the monophasic oral contraceptive containing 100 micrograms levonorgestrel combined with 20 micrograms ethinylestradiol effectively inhibits ovulation, providing adequate suppression of ovarian activity.