The hypothalamic gonadotrophin-releasing hormone (GnRH) stimulates synthesis and secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH) by the gonadotrophic cells of the pituitary. The mechanisms of action of GnRH antagonists and of agonists is completely different. Due tot competitive blockage of GnRH receptors by antagonist administration, LH (and to a lesser extent FSH) levels drop rapidly. Moreover pituitary function normalizes immediately following cessation of medication. The direct and rapid action of GnRH antagonists, the dose dependent suppression of LH and FSH and the rapid restoration of hypophyseal function after cessation of the use of antagonists may shorten and simplify in-vitro fertilization, with less chance of side effects or complications. Further studies are required to decide whether antagonists can usefully be applied for other gynecological indications such as the polycystic ovary syndrome. The possibilities of profitable long term treatment will increase considerably if it proves possible to develop a sustained action formulation.