The concentration of PRL was measured every 20 min for 24 h in six prepubertal and three adolescent boys. In both groups, PRL secretory episodes occurred throughout the 24-h period. In all subjects, the mean concentration of PRL was significantly higher during sleep than during wakefulness; the mean concentration during the entire 24-h period, during sleep or during wakefulness, was not different between the prepubertal subjects and the adolescents. These data suggest the absence of an ontogenetic change for PRL secretion in boys. During acute sleep-wake reversal, two of three pubertal boys showed significantly higher PRL during daytime sleep than during nocturnal wakefulness. This suggest that PRL release in adolescent boys is linked with sleep, rather than with clock time.