Previous studies of raloxifene conducted in animal models and postmenopausal women have demonstrated antiestrogenic action on the endometrium. The purpose of this first study of raloxifene in women with normal menstrual cycles was to determine its reproductive endocrine and endometrial effects. In part I, raloxifene (400 mg) was administered for 5 days in the follicular, periovulatory, or luteal phase of the menstrual cycle (n = 12). In part II, women were randomized to receive raloxifene (100 or 200 mg) for 28 days beginning on day 3 of the cycle (n = 19). All women ovulated in both parts of the study. Raloxifene did not alter the length of the menstrual cycle or the day of the LH surge. A 5-day course of raloxifene administered in any phase of the cycle elevated FSH area under the curve (AUC) for the entire cycle and estradiol AUC for the second half of the cycle compared with those in control cycles. In part II, raloxifene also appeared to increase the FSH AUC and estradiol AUC. Raloxifene decreased the number of gland mitoses in follicular phase endometrial biopsies. Subtle effects suggestive of gland-stromal dysynchrony were noted in a limited number of the secretory phase endometrial biopsies. This study has demonstrated that 1) raloxifene does not prevent ovulation in women with normal menstrual cycles; 2) ovarian estrogen production will continue, and in some cases increase, in response to raloxifene; and 3) antiestrogenic effects of raloxifene on the endometrium are subtle in the endocrine milieu of normal to high circulating estradiol concentrations.