Analogs of GnRH, given chronically in a continuous fashion, produce a paradoxic inhibition of pituitary gonadotropin secretion and, consequently, gonadal steroidogenesis. Thus, GnRH analogs are an attractive class of compounds for achieving a medical castration in the treatment of hormone-dependent neoplasms. In a group of 25 premenopausal patients with progressive advanced breast cancer, daily sc administration of 1-10 mg Leuprolide [D-Leu6-Pro9GnRH ethylamide (NEt)] induced objective tumor regression in 44% with a median duration of 9 months. All women treated for at least 10 weeks developed amenorrhea. Profound suppression of gonadotropins, estradiol, and progesterone secretion occurred in all patients on chronic therapy and persisted for the whole treatment period. These effects on tumor growth and ovarian hormone levels are similar to those observed after surgical ovariectomy. Other GnRH analogs such as Buserelin and Zoladex have been found to have similar antitumor and hormonal effects which are also comparable to those produced by surgical ovariectomy. The mode of drug administration is important. Consistent suppression of ovarian function has only been observed with sc injections of the analogs. Chronic intranasal therapy has been found to induce an incomplete suppression of ovarian function in most patients, probably as a result of the poor absorption of these compounds through this route (approximately 2%). Treatment of metastatic breast cancer with GnRH analogs has been associated with remarkable absence of significant toxicity. Despite some evidence in favor of a direct antitumor effect independent of suppression of ovarian function, the use of GnRH analogs in the therapy of advanced breast cancer should be restricted to premenopausal women.