In order to investigate the possible ovulation inhibitory effect of a new oral contraceptive containing a combination of 20 micrograms ethinyl estradiol and 250 micrograms levonorgestrel peripheral serum from five healthy women between 17 and 24 years were analysed for FSH, LH, estradiol 17-beta and progesterone. The measurements were carried out during a control cycle before the treatment, during two treatment cycles and during a subsequent control cycle. In addition, serum levels of ethinyl estradiol and norgestrel were determined during the treatment periods. Radioimmunological methods were utilized for all the measurements. In all five women studied there was a complete inhibition of ovulation during treatment as indicated by a lack of mid-cycle LH peaks and by suppression of normal luteal phase levels of progesterone. Four out of the five women showed no biphasic estradiol pattern. During treatment the serum levels of levonorgestrel varied between 1 and 10 nmol/l while the levels of ethinyl estradiol usually were below the limit of detection for the method used (below 85 pmol/l). Post-treatment control cycles revealed a re-establishment of the ovulatory pattern in four of the five subjects.