The menstrual cycle reflects the expression of a cyclical process involving the interaction between the hypothalamic-pituitary axis and the ovaries. This complex process requires an integrated neural and humoral control mechanism. It is now well established that a hypothalamic "transducer" located in the medial basal hypothalamus integrates neural and humoral information and translates it into an oscillatory signal which eventually results in the release of the gonadotropin releasing hormone (GnRH), triggering the secretion of gonadotropins from the pituitary gland. Recent animal studies indicate that melatonin influences the functions of the hypothalamic-pituitary-gonadal axis by modifying the firing frequency of the hypothalamic GnRH pulse generator. Consequently, the pineal gland, through the action of melatonin, may exert an important modulatory effect on the mechanisms controlling menstrual cyclicity. Furthermore, abnormal melatonin functions may be involved in the pathogenesis of several disorders of the menstrual cycle including some forms of hypothalamic amenorrhea such as exercise and malnutrition-induced amenorrhea. Consideration of pineal melatonin functions provides a new dimension into the understanding of the neuroendocrine mechanisms governing the cyclical phenomena of the female reproductive system.