Recurrent sustained ventricular tachycardia (VT) with QRS morphology of the right bundle branch block and left axis deviation was studied in 4 patients without any underlying heart diseases. The mean VT rate was 155 beats/min and the endocardial catheter mapping during VT showed the earliest activation site at the left ventricular lateral wall near the apex. In all patients, rapid pacing from the right ventricular outflow tract during VT resulted in constant fusion beats except for the last entrained beat (thus VT was entrained), while pacing from the right ventricular apex and from the earliest activation site failed to demonstrate entrainment. During entrainment from the right ventricular outflow tract (mean pacing rate 168 beats/min), conduction intervals from the pacing site to the earliest activation site (St-A interval) and to the right ventricular apex (St-B interval) were measured in 3 patients. The St-A intervals were 400, 410 and 440 ms and the St-B intervals were 80, 70 and 90 ms, respectively. A small dose of verapamil (1.0 mg) was administered during VT, which resulted in a decrease of VT rate by a mean of 23 beats/min. During entrainment from the right ventricular outflow tract the St-A interval was prolonged in all 3 patients while the St-B interval remained the same. In conclusion, the mechanism of this VT was best explained by reentry with an area of slow conduction. Verapamil slowed the rate of VT by prolonging conduction within the area of slow conduction. Tachycardia entrainment makes possible a selective examination of antiarrhythmic drug effect on the area of slow conduction within the reentry circuit of VT.