Despite variability in the triggers, timing, and pace of sexual maturity between species, all species utilize the final pathway of hypothalamic secretion of gonadotropin releasing hormone (GnRH) to initiate and maintain the reproductive axis. Thus, GnRH is required for reproductive competence in the human. The classic studies from the 1970s clearly demonstrate that pulsatile release of GnRH from the hypothalamus is a prerequisite for physiologic gonadotrope function. Absence, decreased frequency or decreased amplitude of pulsatile GnRH release results in the clinical syndrome of hypogonadotropic hypogonadism (HH). The phenotypic expression of GnRH deficiency in the human demonstrates considerable heterogeneity. Defining the physiology of GnRH is critical to understanding the clinical heterogeneity of isolated GnRH deficiency and its comparison to other conditions resulting in hypogonadotropic hypogonadism (HH). The overall goal of this protocol is to investigate the neuroendocrine control of reproduction and specifically the physiology and pathophysiology of GnRH secretion and action in the human male.