When ventricular pacemakers are electrically stimulated at a fast rate, the cessation of the drive is followed by a temporary suppression of automaticity ('overdrive suppression'). The role of the sympathetic system in the regulation of this overdrive suppression was studied and the following results obtained: Hypotension induced during overdrive by amyl nitrite inhalation caused a shortening of the pause which follows the drive. Bilateral resection of the stellate ganglia led to the opposite effect. Maximal stellate ganglion stimulation halved the pause duration. Norepinephrine infusion accelerated the idioventricular rate and shortened the pause. Beta adrenergic blockade decreased the idioventricular rate and prolonged the pause. It is concluded that the tonic sympathetic discharge not only controls the basal idioventricular rate but also plays a regulatory role in the suppression of idioventricular automaticity which follows overdrive. This regulatory role can be dissociated from changes in idioventricular rate prior to overdrive.