The effect of vagus nerve stimulation on ventricular excitability was studied in 28 dogs under various conditions of adrenergic neural tone. Strength-interval curves were delineated from the apex of the right ventricular endocardium with a transvenous bipolar catheter. Vagus nerve stimulation in both closed chest and open chest dogs shifted the strength-interval curve 6 to 8 msec later into electrical diastole (P less than 0.001). Left stellate ganglion stimulation shifted the strength-interval curve 9 to 11 msec earlier into diastole (P less than 0.001). The effect of simultaneous left stellate ganglion and vagus nerve stimulation was not significantly different from that of left stellate ganglion stimulation alone. The influence of vagus nerve stimulation on the strength-interval curve under basal conditions was abolished by acute beta adrenergic blockade with propranolol. It is concluded that vagus nerve stimulation affects ventricular excitability as well as vulnerability by opposing the effects of sympathetic neural tone.