1. 1. Cortical temperature was measured with subdural thermistors during waking and sleep in eight Macaca mulatta monkeys. A normal diurnal curve was present, and during the day periods of heightened arousal were associated with temperature increases. During the night periods of restlessness or awakening were associated with temperature increases, the temperature decreasing as the animal returned to sleep. 2. 2. REM sleep was most often associated with decreases in cortical temperature, irrespective of whether the animal entered REM sleep from non-REM sleep or from waking. In general, thermistors posterior to the central sulcus or over temporal cortex recorded statistically significant temperature decreases during REM sleep, whereas thermistors over frontal cortex recorded either significant decreases or no significant change with REM. No recording sites showed significant temperature increases during REM sleep. 3. 3. It is suggested that these results represent increased cortical blood flow during REM sleep, and that cortical blood flow during REM sleep may exceed flow during non-REM sleep, quiet waking or intense arousal.