The pineal gland, through its major secretory product melatonin, influences seasonal breeding in species such as the hamster and the sheep. Recent studies from our laboratory have shown that melatonin also affects sexual development in the rat. A role for melatonin in humans has not yet been found. The laboratory rat is sensitive to daily administration of melatonin at the beginning of sexual maturation. The male rat is most sensitive between day 20 and day 30 of life. Melatonin does not permanently inhibit sexual maturation, since normal but delayed sexual development occurs after 45 days of life whether melatonin administration is discontinued or maintained indefinitely. In female rats, daily injection of melatonin during the prepubertal period delays the vaginal opening and disrupts the normal cyclicity of the first oestrous cycles. In both male and female rats, the inhibitory action of melatonin is highly dependent upon the time of injection, with maximal effects when melatonin is given in the late photoperiod. The inhibitory action of melatonin is most likely exerted at the hypothalamic level, possibly through interference with the control of pulsatile secretion of gonadotropin-releasing hormone. In contrast to some published work, our experiments provide no evidence for modifications of diurnal or nocturnal melatonin secretion during puberty in humans. Our results with the rat indicate that melatonin may be an important factor for the timing of sexual maturation.