Two groups of unanesthetized dogs, each consisting of six animals with ventricular tachycardia caused by two-stage ligation of the anterior descending branch of the left coronary artery on the day before treatment, were given continuous intravenous infusions of 0.5 mg/kg/min of either droxicainide [DL-N-(2-hydroxyethyl) pipecolinyl-2,6-dimethylanilide] hydrochloride or lidocaine hydrochloride until convulsions occurred. At low cumulative doses of either drug, the infusions produced a progressive reduction in the frequency of ventricular ectopic beats, but no significant changes in the sinus rate, the PR or QRS intervals of normal sinus beats, the arterial and central venous pressures, or the respiratory rate. At higher cumulative doses lidocaine produced sedation, and both drugs produced emesis and then convulsions that subsided soon after termination of treatment. With respect to both the cumulative doses and plasma concentrations at which they reduced the frequency of ectopic beats by 25, 50, and 75% and at which they produced convulsions, droxicainide was a more potent antiarrhythmic agent than lidocaine and had a wider margin of safety.