Single neurons were isolated in the cuneate and gracile nuclei of the cat. The effects of electrical stimulation of the surface of the cerebral cortex on the excitability of these neurons were determined by measuring the change in their responsiveness to cutaneous electrical and natural stimulation at various times following cortical conditioning stimulation. The following conclusions have been drawn. The peripherally evoked activity of neurons in the dorsal column nuclei can be either facilitated or depressed, but rarely both, by stimulation of the sensorimotor cortex; other areas of cortex are not effective. The mechanism of the depressive influence is that of active inhibition and does not involve an occlusive mechanism; this appears to be true for all neurons affected in the dorsal column nuclei. Stimulation of the cortex contralateral to the site of recording produces a more powerful inhibition than does comparable stimulation of the ipsilateral cortex. Not only can evoked activity be inhibited, but on-going, spontaneous activity can be depressed by cortical stimulation. The cortically originating inhibition can be graded according to the relative strength of the cortical conditioning and peripheral testing stimuli.