The success of pacing stimuli interruption of ventricular tachycardia (VT) was examined in 77 episodes of sustained VT induced in 31 patients undergoing programmed electrical stimulation studies. Once VT was induced, a trial to terminate the arrhythmia by means of the technique of entrainment was attempted. If this failed, rapid burst pacing faster than the VT was begun to try and terminate the tachycardia. In 30 patients off antiarrhythmic agents, entrainment was effective in terminating VT in 27%, while burst pacing was also effective in 27%. In 37% of patients, VT was accelerated or ventricular fibrillation was produced by pacing techniques, and these patients required defibrillation. Following antiarrhythmic therapy that failed to prevent VT induction but did result in slowing of VT rate, entrainment was only successful in 23% of trials, while burst pacing was successful in 34% of trials. The incidence of acceleration of VT on therapy was 32%. There was no appreciable difference in acceleration noted with or without antiarrhythmic therapy. Regardless of therapy, the slower the VT rate, the greater success of pacing termination of VT and the lower the incidence of VT acceleration. Antiarrhythmic agents that significantly slow the VT rate increase the success rate of pacing stimuli interruption of VT and decrease the incidence of VT acceleration and thus the need for defibrillation. The results suggest that antiarrhythmic agents that slow the VT rate may increase the effectiveness of antiarrhythmic pacemakers in terminating VT.