Experimental ventricular tachycardias (VT) were provoked in dog hearts with minute crystals of aconitine introduced into the periphery of an infarcted area, produced by intramural injection of 1-1.5 ml of phenol. The response of these tachycardias to verapamil was studied. Leads II, aVL, intraventricular right and left unipolar records, as well as one on the superior vena cava were registered under control conditions, with VT and after the injection of the antiarrhythmic agent. This injection was infused into superior vena cava over 15-20 minutes. Records were obtained with constant intervals, waiting for the recovery of sinus rhythm (SR) and the posterior reappearance of ventricular tachycardia. The experiments were performed 6 to 8 hs with continuous infusion of Hartmann's solutions. Throughout these periods, the variations of systemic systolic pressure were registered. Of the 75 dogs treated with 0.2 mg/kg of verapamil, SR was recovered transiently in 30 (40%), while it exerted no antiarrhytmic effect in 19 (25%). Arterial systolic pressure fell importantly in 10 animals (13%). In 3 other groups, of 15 dogs each, comparative administration of verapamil vs lidocaine (I), vs mexiletine (II) and vs propafenone (III), was tried. In Group I, verapamil reestablished transient SR in 73% and lidocaine in only 7%; in II, SR resulted from verapamil in 33% and from mexiletine in 7%; in III, SR reappeared in 21% with verapamil and in 28% with propafenone. The repeated positive effect of verapamil was found in 33% of 15 experiments. This drug is effective in certain experimental ventricular tachycardias, probably related to calcium-dependent potentials.