The alterations in hormone production that accompany human pregnancy are the most remarkable recorded in human physiology or pathophysiology. In pregnant women, at or near term, there is the daily production of 15-20 mg estradiol-17, 8 (E2), 50--100 mg estriol (E3), 250--600 mg progesterone (P4), 1-2 mg aldosterone, and 3-8 mg deoxycorticosterone (D OC). Fur thermore, there are strikingly increased levels of plasma renin, angioten sinogen, and angiotensin II, and the daily production of I g of human placental lactogen (hPL), massive quantities of human chorionic gonado tropin (hCG), and likely human chorionic thyrotropin (hCT), chorionic ACTH, and possibly chorionic TRH, LHRH, and somatostatin. The most remarkable physiologic phenomena of pregnancy may be the mechanisms by which the woman and her fetus adapt to this unusual endocrine milieu. Many of the hormones cited above are produced by the placenta. The placenta secretes large quantities of a variety of both steroid and polypep tide hormones. These are the topic of this review.