The chemical structure of luteinizing-hormone-releasing hormone (LHRH) was discovered in 1971 after more than a decade of intensive effort. Subsequent physiologic studies in primates and humans showed that the biologic activity of LHRH depends on the way in which the hormone is administered. Pulsatile administration of LHRH, which mimics the natural secretory pattern, causes sustained secretion of the gonadotrophins. This method of administration has been used to induce ovulation in women with hypothalamic amenorrhea and to induce puberty and spermatogenesis in men with hypogonadotrophic hypogonadism. Continuous infusion, however, produces only transient stimulation of gonadotrophin secretion, followed by a "desensitization" response in which gonadotrophin secretion is inhibited. Thus, LHRH can either augment or inhibit gonadotrophin secretion depending on the mode of administration. Recently, long-acting synthetic analogs of LHRH have been shown to desensitize the pituitary gland and inhibit gonadotrophin release when administered as a single daily subcutaneous injection. These LHRH analogs have proved highly effective in the treatment of prostatic carcinoma and central precocious puberty. They are also being studied as a new approach to contraception and to the treatment of endometriosis and polycystic ovary syndrome.