On 2 consecutive nights, plasma LH, FSH and testosterone (T) were measured every20 min for 12 h during evening wakefulness and polygraphic sleep in 5 pairs of male monozygotic twins in pubertal stages 1-4, and in a male dizygotic also studied in 3 twins. During sleep, significant enhancement of episodic LH release was seen on 16 of 18 nights on the stage 1-4 twins. During wakefulness, minimal episodic LH release was observed in the stage 1-3 twins, which then gradually increased in the more mature twins, until finally the significant sleep-wake difference in mean LH was lost in the stage 5 male. Testosterone also rose significantly in sleep on 19 of 20 study nights in the stage 1-5 twins. In the early pubertal twins this nocturnal rise in T was small, but in the midpubertal pairs it was profound, as peaks in T occurred which lay in the normal range for adult males. In these less mature twins the majority of the episodic secretion of T also was limited to sleep. In wakefulness, the T levels gradually increased across puberty until, in the stage 5 twin, wakeful peaks in T finally reached the adult male range. In the midpubertal twins, a close temporal relationship was seen between initiation of sleep-enhanced LH release and the subsequent initial rise in T (mean lag time 29.1 min). In the stage 5 twin, this episodic LH-T relationship persisted into wakefulness where the largest increments in T were seen just prior to sleep onset. Evidence of sleep-enhanced FSH release was more equivocal, and was limited mainly to pubertal stage 1 and 3 pairs. Similarities in hormonal patterns were seen within the monozygotic twin pairs and probably contributed to the parallel progress in puberty of the pair. Thus, sleep-wake rhythmicity in release of gonadotropins, particularly LH and thereby of testosterone, was seen to evolve transiently in twin boys across puberty. The existence of such rhythmicity suggests that a fundamental, sleep-entrained CNS mechanism plays an important, if not a dominant, role in sexual maturation in boys.