Most of the following chapters were presented as plenary lectures or symposium talks at the 1986 XXXth Congress of the International Union of Physiological Sciences in Vancouver, B.C. A distinguished international group of endocrinologists and physiologists have contributed up-to-date reviews of their particular fields. The early chapters are largely concerned with the brain and neuroendocrine mechanisms controlling the secretion of gonadotropin releasing hormone (GnRH) and its action on the anterior pitui- tary gland. Later chapters focus on the gonads themselves and the systemic and intrinsic hormones influencing the functional cytology of ovarian and testicular cells. Such comprehensive subjects as sex differentiation, puberty, placentation and parturition are also discussed authoritatively. According to Pfaff and Cohen and Arai et al., gonadal steroids, especially estrogen, exert multiple effects on certain hypothalamic and preoptic neurons, including growth, protein synthesis and electrical changes, which promote plasticity and facilitate synaptogenesis. The electrophysio- logy of the hypothalamic GnRH pulse generator in the rhesus monkey is reviewed more specifically by Knobil. In ovariectomized ewes, Clarke finds both positive and negative effects of estrogen on hypothalamic release of GnRH as well as on pituitary responsiveness to the peptide. Flerk6 et al.