We compared the effects of verapamil to high dose procainamide on the rate of inducible and spontaneously occurring ventricular tachycardia (VT) in 10 patients. Verapamil induced a significant increase in the rate of tachycardia (R-R interval decreased from 278 +/- 54 to 233 +/- 32 ms, mean +/- SD; p less than 0.025 by paired t test) while procainamide slowed the tachycardia (mean R-R interval was 328 +/- 72 ms, p less than 0.02). Verapamil prevented the induction of sustained VT and was effective as chronic oral antiarrhythmic therapy in 2 patients. The accelerated VT culminated in ventricular fibrillation in 1 patient. It is assumed that verapamil may have either increased conduction velocity or shortened the reentrant cycle. This may be related either to a primary effect of the drug or secondary to increased catecholamine stimulation due to a vasodilatory effect.