In the United States 10-15% couples seek evaluation for infertility, of whom a male factor is identified in approximately 50% of cases. To date, however, most infertility research has focused on the female partner and in the majority of instances the etiology of male infertility remains poorly understood with inconsistent elevation in FSH levels. Given that spermatogenesis is highly dependent on FSH secretion, this project will focus on delineating the feedback control of FSH in the human male. Such studies, although difficult to perform in the human, are critical to understanding not only the pathophysiology of male infertility, but also to determining the feasibility of hormonal approaches to male contraception. In this protocol, studies of normal, GnRH-deficient and agonadal men employing reversible sex steroid ablation, physiologic sex steroid replacement and manipulation of the GnRH input to the pituitary will permit the non-steroidal, sex steroid, and GnRH-dependent components of FSH regulation to be selectively elucidated.