The effects of estradiol-17β (E2) and clomiphene citrate (CC) on pituitary-uterine function in ovariectomized (OVX) rats were compared in two experiments. In experiment 1 CC completely blocked E2 stimulation of uterine weight when administered concurrently for 10 and 30 days. In experiment 2, content of total (occupied and unoccupied) uterine nuclear estrogen receptors was increased by E2, but not by CC, compared to OVX controls. Only a fraction of the nuclear receptors was occupied in E2-treated rats, whereas all of these receptor sites were occupied in CC-treated rats. In addition, uterine cytoplasmic estrogen receptors were increased by E2 and diminished by CC treatment. Both E2 and CC were effective in preventing the postovariectomy rise in serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH), although LH was always inhibited to a greater extent than FSH. After 30 days of treatment, pituitary gonadotropins were also substantially reduced by E2 and CC, suggesting that long-term treatment decreased both pituitary synthesis and release. An increase in pituitary weight observed in E2-treated rats only was due largely to the stimulation of lactotropes, as serum prolactin was increased. In summary, although CC and E2 similarly depressed pituitary gonadotropin secretion, they exhibited marked differences in the stimulation and occupancy of uterine estrogen receptors and the stimulation of pituitary prolactin secretion.