The results of antiarrhythmic testing in a variety of animal models indicate that flecainide acetate has potent antiarrhythmic activity. In these models it is more potent than lidocaine, procainamide and quinidine, as well as a number of investigational agents, and is active against both ventricular and supraventricular arrhythmias from a number of causes. Studies of its effects on the action potential from various cardiac tissues indicate that its primary effect is to slow the rate of rise of the action potential, placing it among the group of drugs commonly referred to as class I agents. In the intact animal, in accordance with its effects on the action potential, flecainide slows conduction throughout the cardiac conduction system, with the most marked effects on His-Purkinje conduction and ventricular activation.