Clearly a major endocrine organ in its own right, the placenta is known to produce de novo at least three protein hormones that may be considered analogues of pituitary hormones and, drawing on precursors from mother and fetus, at least such steroid hormones as estrogens and progesterone. The full picture is only now beginning to be drawn, however, and much remains to be learned of the physiologic roles played by the substances the placenta elaborates.