The specific activity of NAD+-dependent 15-hydroxyprostaglandin dehydrogenase was measured in human endometrial tissue obtained from ovulatory and anovulatory women. Employing PGE2 as substrate, the specific activity of this enzyme was found to be highest in endometrial tissue during the secretory phase of the cycle (ovarian cycle days 15-25) and lowest in menstrual (days 1-5) and premenstrual (days 26-28) endometrium. The specific activity of prostaglandin dehydrogenase in endometrium of anovulatory women was low, being similar to that found in proliferative endometrium (days 6-14) of ovulatory women. Prostaglandin dehydrogenase activity was found in the cytosolic fraction prepared from endometrial ti-sue, and was found principally in the glandular epithelium following separation of endometrial glands and stromal cells.