Medroxyprogesterone acetate (Provera) was administered orally during 2–4 days to women undergoing endometrial curettage during the follicular phase of the menstrual cycle. Estradiol (E2) receptor levels were estimated from the amounts of 3H–E2 tightly bound in nuclei after incubation of endometrial tissue with an excess of the labeled hormone. An average value (9 subjects) of 1.5 pmole E2/mg DNA ± 0.7 (mean ± SD) was found. This value is significantly lower than the average E2 receptor levels in proliferative endometrium of untreated subjects (3.2 ± 1.3, n = 33) and equals the level observed in the early secretory phase (1.5 ± 0.5, n = 5). These results indicate that one of the progestin effects on human endometrium is the reduction of E2 receptor levels.