Preliminary clinical experience indicates that flecainide is a useful antiarrhythmic agent for the suppression of spontaneous ventricular arrhythmias. The drug also exerts marked effects on accessory atrioventricular (AV) pathway and retrograde fast AV nodal pathway refractoriness, and therefore may be effective in treatment and prevention of reentrant supraventricular tachycardias. Electrophysiologic studies indicate that flecainide slows atrial, AV nodal, His-Purkinje and intraventricular conduction and, to a far lesser degree, prolongs refractory periods in these tissues. The effect of the drug in slowing conduction within the His-Purkinje system is particularly marked and is commonly associated with prolongation of the HV interval beyond the normal range. Flecainide causes a small but significant increase in the QT interval duration, which results largely from prolongation of the QRS interval. Significant prolongation of the sinus node recovery time has been observed in patients with preexisting sinus node dysfunction. Available electrophysiologic data suggest that flecainide should not be administered to patients with advanced disease of the His-Purkinje system or sinus node. The safety of the drug in patients with mild to moderate abnormalities of His-Purkinje conduction or sinus node function awaits further study.