The paradoxical association of relatively large amplitude alpha 10± per second activity with a relatively low level of waking cerebral function is attributed to the inhibitory effects of subcortical 10 ± per second pacing of activity in the cortex and to associated autonomic effects causing cerebral constriction. On the other hand the association of fast activity with cortical excitation, metabolite (CO2) production, and fast activity in cholinergic nerves accounts for associated vasodilatation. Thus, by the opposed effects of fast vs. 10 ± per second slower activity, cortical and subcortical mechanisms may regulate one another. This may provide a homeostatic mechanism by which excitation in the cortex is normally prevented from producing excessive discharge or from self-perpetuation. Underactivity, overactivity, or imbalance of these functions would account for the EEG and autonomic patterns seen in many abnormal mental conditions.