The hormonal activity of the pineal gland is influenced by both the dark-light cycle and the seasonal cycle, causing it to play an important role in the neuroendocrine control of reproductive physiology. This is especially evident in seasonally breeding animals, in which reproductive function is clearly influenced by seasonal variations in the duration of night and day. Humans are not seasonal breeders. Nevertheless, seasonal fluctuations have been described in human reproduction, and the pineal gland also appears to exert an important role in the neuroendocrine regulation of human reproductive physiology. There is evidence that the epiphysis is involved in the control of sexual maturation. In rats, the maternal pineal appears to influence the gonadal and genital development and function of offspring; this hypothesis has yet to be confirmed in humans. The pineal apparently influences human reproductive function not only at the hypothalamic-pituitary level, by inhibition of the hypothalamic pulsatile secretion of gonadotrophin-releasing hormone, but also at the gonadal level, where melatonin receptors have also been found. In addition, melatonin is reported to increase serum prolactin concentrations in both rats and humans. It has been suggested that melatonin is involved in the control of menstrual cyclicity.