Luteal cytoplasmic estradiol (E2) and progesterone (P) receptor levels were measured from the 22nd to the 25th days of the menstrual cycle in endometrial samples obtained from seven patients in an in vitro fertilization (IVF) program who received no embryo replacement after ovarian stimulation with clomiphene citrate/human menopausal gonadotropin/human chorionic gonadotropin, and from seven normally menstruating women. Serum levels of E2, P, follicle-stimulating hormone, luteinizing hormone, and prolactin (PRL) were measured in blood samples collected at the time of biopsy. The E2 (P less than 0.01) and PRL (P less than 0.001) levels were higher in stimulated than in spontaneous cycles. The level of cytoplasmic P receptor was decreased in endometrium in stimulated cycles, but cytoplasmic E2 receptor remained unchanged. These alterations in the luteal phase of cytoplasmic P receptor in the endometrium could be involved in the low rate of success following the IVF program.