The data here presented indicated that stimulation of the brain stem reticular formation is accompanied by vascular changes leading to increase of cortical temperature. From comparison with other factors whose influence on cerebral vessels is known, it seems likely that either increase in blood flow or vasodilation may occur. Such vascular responses generally, but not always, accompany arousal EEG patterns. The study of possible neurohumoral mechanisms responsible for such phenomena is complicated by many factors. My results indicate that acetylcholine rather than adrenergic compounds may be involved.