P lasma luteinizing hormone (LH) and testosterone (T) were measured by radioimmunoassay in nine pubertal boys and three sexually mature young men at 20-min intervals for 24 h. Plasma LH and T were also measured in one boy during a delayed sleep onset study. Polygraphic monitoring was carried out to identify precisely sleep onset. Wakefulness, and specific sleep stages. In all nine pubertal boys the plasma T concentration fluctuated and was significantly higher during normal nocturnal sleep as compared to daytime waking. This increased T secretion during sleep was temporally linked to the characteristic pubertal sleep augmentation of LH secretion. To define further the relationship of this increased T secretion to sleep, plasma LH and T were also measured in three of the pubertal boys after acute (1-day) reversal of the sleep-wake cycle. One of these boys was also studied after 3 days of sleep-wake cycle reversal. The results of these studies showed that plasma T was now augmented during the reversed daytime sleep period; the mean T concentrations during this period were significantly higher (P < 0.001) than during nocturnal waking in all four studies. Measurement of plasma LH and T in the three sexually mature young men showed episodic secretion of LH and T during both waking and sleep periods; there was no consistent significant augmentation of LH or T secretion during sleep. This study demonstrates that (a) in normal pubertal boys and sexually mature young men plasma T fluctuates episodically; (b) there is marked augmentation of T secretion during sleep in pubertal boys, which is dependent on increased LH secretion; (c) this pubertal LH-T secretory “program” is dependent on sleep, since it shifts with delayed sleep onset and reversal of the sleep-wake cycle; and (d) this demonstrable tropic effect of LH on T is evident only during puberty, since sexually mature young men fail to show any consistent relationship between LH and T secretion either awake or asleep.