A symmetrical suppression of high voltage, slow wave activity was noted in tracings taken at the first and fourth hours. At the twenty-fourth hour, the patterns had returned to pre-test levels in six of eight patients. It has been demonstrated that the high voltage bursts and slow wave activity appearing after electric convulsive treatment originated in the diencephalon. Since mescaline suppresses these slow waves, the diencephalon would appear to be one of its areas of action.