The chemical characterization of the hypothalamic decapeptide gonadotropin-releasing hormone (GnRH) has stimulated the development of analogues of GnRH with important clinical applications. Chronic administration of the GnRH analogues nafarelin acetate and leuprolide acetate results in an initial stimulation of pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion, which is followed by a profound decrease in the secretion of LH and FSH. The decrease in the secretion of LH and FSH produces a hypogonadal state that is associated with an improvement in many sex-steroid-dependent disease processes. The GnRH analogues are clearly effective in the treatment of prostate cancer, endometriosis, uterine myomas, polycystic ovarian disease, and the premenstrual syndrome.