O-demethylated Encainide (MJ 9444) and 3-O-methoxy Encainide (MJ 14030), on cardiac conduction is studied by recording His bundle potentials in isolated perfused rabbit hearts and Purkinje and muscle conduction in vivo in dog hearts after destruction of the atrioventricular node. Both metabolites are 4 to 15 times more potent than Encainide in slowing conduction through the atria, the AV-node and the His-Purkinje system of the rabbit heart. In the dog, Encainide (0.8-3.2 mg/kg i.v.) slows conduction of extrasystoles in both Purkinje and muscle at all coupling intervals, increases the effective refractory period and the functional refractory period of the Purkinje pathway[1].