Endometrial acid and alkaline phosphatases were studied histochemically in rhesus monkeys treated with various combinations of estrogen (E, 17β-estradiol and/or estriol), progesterone (P) and relaxin (R) or a low potent relaxin control preparation (NRF). In the cells of the uterine glands of the E-treated animal, the apical cytoplasm showed intense activity of both phosphatases. This estrogenic response was depressed in the stratum functionale by P and in the stratum basale by PR. With E, acid phosphatase-staining granules appeared in scattered stromal cells with eccentric nuclei. Addition of P or PR increased the number of acid phosphatase positive stromal cells, especially in the stratum functionale. With the exception of the sinus-like channels and superficial vessels of the stratum functionale of monkeys treated with ER and EPR, all endothelium of capillary and precapillary vessels was rich in alkaline phosphatase activity. Thus, acid alkaline phosphatases appear to be metabolically important components of the endometrium which undergo cyclic variation and reflect specific influences of the ovarian hormones; estrogens, progesterone and relaxin. The implications to human menstrual physiology are discussed.