Plasma concentrations of cortisol (F) and growthhormone were determined in samples obtained sequentially at 15 min intervals for 12 h night and 12 h day sessions in juvenile male monkeys (6–9.5 lb.) implanted with right atrial catheters. Electroencephalographic (EEG), electrooculographic (EOG), and electromyographic (EMG) activity were recorded. Cortisol was secreted episodically, increasing in frequency and concentration during the latter portion of the night and early morning hours. Growth hormone was also secreted episodically but bore no apparent relationship to either sleep stage or time of da