The aim of the present study was to investigate the effects of tamoxifen on pituitary and luteal function and on the bleeding pattern when administered continuously in the secretory phase. The study included 16 women with regular menstrual cycles followed during one control, one treatment and one follow-up cycle. Each volunteer received 20 mg tamoxifen twice daily from cycle day 18 to menstruation in the treatment cycle. The luteal phase was slightly, but significantly prolonged during treatment, and FSH, progesterone, 17-hydroxyprogesterone, 20α-dihydro progesterone, estrone, estrone sulphate and estradiol significantly elevated in comparison with corresponding data during the control cycle. The results indicate that estrogen may be of some importance for the regulation of the life span of the corpus luteum in the human. The significantly elevated levels of pregnanediol glucuronide and estrone glucuronide during the follow-up cycle are most likely a result of either a direct effect of remaining circulating tamoxifen levels on the ovary, or mediated through the increased release of FSH. If estrogens are of importance for the process of implantation, which has recently been suggested in subhuman primates, also in the human remains unclear. Studies on the effect of anti-estrogens on the endometrium during the secretory phase of the cycle are ongoing.