I. Introduction GONADOTROPIN-releasing hormone (GnRH)1 is a key integrator between the neural and endocrine systems (Fig. 1). This peptide is synthesized in hypothalamic neurosecretory cells then released in a pulsatile pattern into the hypothalamo-hypophyseal portal circulation. This pulsatile pattern provokes the secretion of the gonadotropins, LH and FSH, from the anterior pituitary. Gonadotropins, in turn, control the function of the gonads; LH stimulates ovulation and corpus luteum formation in females and androgen secretion in males, whereas FSH stimulates the growth and maturation of ovarian follicles in females and spermatogenesis in males. GnRH and its analogs also have direct actions on the gonads in some species by inhibiting steroidogenesis in males and ovulation in females (1). It is unlikely that hypothalamic-derived GnRH is responsible for these actions (because of the very low concentrations in the peripheral circulations), and the possibility remains open that a different molecule, made locally, may bind to this receptor in vivo. The ability of GnRH to stimulate reproductive functions (at pulsatile low doses) or suppress them (at high doses) has been clinically applied for various purposes (2), including the induction of ovulation and spermatogenesis, contraception, and for the treatment of precocious puberty, endometriosis, and steroiddependent tumors (i.e. prostate and breast cancer).