Synchronization of GH-secretory episodes occurs at light onset in the rhesus monkey and is later lost during the 24-h day. The stimulus for this synchronization was studied in nine male adult monkeys prepared with chronic jugular catheters. In a first experiment, light onset was delayed by 2 h (from 0600 to 0800 h) on days 2-6 and returned to 0600 h on day 7. The animals were allowed to sleep on during the additional time of darkness. The early morning GH peak was immediately shifted to the new light-onset times, both after introduction of the delay and after its reversal. In a second experiment, the animals were awakened in darkness during a similar light-onset delay. Now the GH peaks constantly occurred at the time of awakening shortly after 0600 h, and no GH peak was seen after light onset at 0800 h. Neither experimental condition had an influence on the patterns of plasma PRL and cortisol. These results suggest that the early morning synchronization of the GH-secretory pattern in the rhesus monkey is triggered by awakening and not by light onset.