In female rats there is a temporary afternoon rise in prolactin during the transition from the weaning period to the period just preceding puberty. The purpose of this study was to determine whether a similar temporary prolactin peak occurs in males at the same age and to assess the dependence of this elevation upon gonadal status and light cycle cues. Blood samples were collected at 08.00, 20.00 h colony time during the prepubertal period. A prolactin peak occurred only at 28 days of age in males. This peak did not occur in either males or females gonadectomized at 22 days of age. Animals reared in constant light from weaning (21 days of age) failed to show an afternoon prolactin peak at 28 or 32 days of age. Animals in constant light from weaning age showed maximum prolactin at 08.00 h on days 28, 30, and 32 with minimum levels at 20.00 h. It can be concluded that both males and females exhibit a transient peak of prolactin during the prepubertal period. This peak was abolished by gonadectomy and by constant light, suggesting that it is an expression of a clock mechanism sensitive to photoperiod and gonadal steroids. The prolactin peak does not appear to correlate with pubertal onset per se since both vaginal opening and balanopreputial separation are precocious in constant light despite the fact that the prolactin peak is abolished in the population as a whole.