The purpose of this review is to summarize the experimental data establishing the baboon as a non-human primate model for the study of the endocrinology of human pregnancy, and to outline the results of in-vivo experiments in the baboon which show that oestrogen plays a central integrative role in the regulation of placental steroidogenic maturation as well as the function and maturation of the fetal adrenal gland. Thus, oestrogen regulates the receptor-mediated uptake of low density lipoprotein-cholesterol and the P450 cholesterol side-chain cleavage system within syncytiotrophoblasts to promote the production of progesterone. Oestrogen concomitantly acts on the fetal adrenal gland to modulate the production of androgen precursors which ensures maintenance of physiologic levels of oestrogen during the course of gestation. In addition, oestrogen regulates the 11beta-hydroxysteroid dehydrogenase enzymes controlling placental cortisol-cortisone metabolism and their secretion into the fetal circulation and thus indirectly regulates the fetal hypothalamic-pituitary-adrenocortical axis. Collectively, these oestrogen-regulated processes ensure the maintenance of pregnancy and the maturation of the fetus including the development in utero of adrenocortical self-sufficiency essential for neonatal survival.