Overdrive pacing has been applied in 26 patients to prevent frequent recurrent ventricular fibrillation (VF) and ventricular tachycardia (VT) occurring in the setting of ventricular extrasystole of 2-5 degrees graded by Lown. These patients had 3-47 recurrent attacks of VF and VT (11.4 +/- 2.4) which were not prevented with antiarrhythmic agents. Overdrive pacing was continued for 2-236 hours (21.3 +/- 3.7) and appeared to be effective in 23 (88.4%) of the 26 patients including those with prolonged QT intervals. Atrial pacing was more effective than ventricular overdriving and required stimulation at a slower rate. Antiarrhythmic therapy and overdrive pacing in combination were more effective than both used independently. Suppression of ventricular extrasystole and prevention of life-threatening arrhythmias were achieved by increasing the heart rate by 23.2 +/- 4.5 beats/min.