Studies were undertaken with bepridil, a new calcium blocker that prolongs the QT interval, to determine the antiarrhythmic and possible arrhythmogenic properties of this agent. The technique of programmed electrical stimulation was employed to evaluate bepridil in 15 patients with symptomatic ventricular tachycardia (VT). Bepridil prevented VT induction in 7 of 15 patients. Bepridil prolonged the QT and refractoriness and a linear correlation could be demonstrated between the percent change in QTc and refractory period prolongation for the bepridil-protected group. Bepridil in one patient reduced by one the number of stimuli required to induce VT, but no spontaneous arrhythmias were noted. Bepridil thus possesses antiarrhythmic properties with a minimal proarrhythmic effect.