The influence of the new antiarrhythmic agent Propafenon on cardiac conduction and sinus node function was studied by using His-bundle recordings and atrial stimulation in 14 patients with normal and diseases conduction system. Intravenous administration of Propafenon in therapeutic dose (1-2 mg/kg) produced a significant prolongation of the atrioventricular conduction time. Increase of the A-H interval was observed in 13 of 14 subjects during sinus rhythm. Second degree A-V block (Wenckebach form) during atrial stimulation occurred at lower frequencies after administration of the drug. The impulse propagagion within the His-Purkinje system was depressed significantly (H-V interval in 8, H-S interval in 10 of 14 subjects). Propafenon did not cause any alteration in intraatrial conduction, but depression of the sinus node automaticity was noted. Total reversal of the drug induced prolonged atrioventricular conduction and a decrease of the sinus rate was seen after administration of orciprenaline. Beta-adrenergic receptor blocking and local anaesthetic direct membrane actions are discussed as possible cause of the prolongation of atrioventricular and intraventricular conduction.