The classical concept of the ascending reticular activating system was reviewed basing upon recent neuroanatomical findings and two kinds of physiological studies, such as recording from brainstem neurons in the discrete nuclei and stimulation of the brainstem nuclei influencing thalamic neuronal activity. We propose that the reticular activating system is not a single system, but is composed of two systems arising from the brainstem: the noradrenergic projection conveying a rather tonic control, and the cholinergic projection responsible for the phasic aspects of the activating system.