A 28-year-old man is described with no demonstrable organic heart disease and recurrent paroxysmal attacks of sustained ventricular tachycardia. Lignocaine and ajmaline failed to terminate the first attack but a bolus infection of verapamil succeeded. This drug was subsequently successful on six more occasions. During electrophysiological study of the eighth attack, slow intravenous administration of verapamil significantly reduced the rate of the tachycardia and prevented its subsequent reinitiation by pacing. Two mechanisms are postulated to explain both the arrhythmia and the beneficial effects of verapamil in this case.