The effects of stellate ganglion stimulation on idioventricular automaticity were studied in anesthetized dogs. An idioventricular rhythm was obtained in one series of animals by ligating the His bundle and in the other by stimulating the vagus. On isolation of the left stellate ganglion, the idioventricular rate fell by 5 beats/min which is compatible with a rate of “tonic” discharge of about 3 impulses/sec. On stimulation of the stellate ganglia at increasingly higher frequencies, the idioventricular rate accelerated progressively and the frequency-response curve was sigmoid. When the stimulation frequency was increased above 10 to 15 pulses/sec, there was no further enhancement of ventricular automaticity. The ventricular rates attained during sympathetic stimulation rarely exceeded 70/min. The interval between the initiation of sympathetic stimulation and the maximal ventricular acceleration was shorter at higher stimulation frequencies. The effects of right and left stellate stimulation on the rate of idioventricular pacemakers were comparable. Reflex vagal inhibition had little influence on the acceleratory action of the sympathetic nerves on the idioventricular pacemakers since in dogs with atrioventricular block the results before and after vagotomy were similar. Furthermore, the idioventricular rhythm during vagally induced block increased on sympathetic stimulation to values comparable to those obtained in dogs with surgically induced block.