Pilsicainide is marketed in Japan as (Sunrythm). Pilsicainide controls the cardiac conduction abnormalities and improves the symptoms of arrhythmia with the blocking action on Na channel. It is usually used to treat tachycardiac arrhythmia. Pilsicainide did not affect the resting membrane potential. Under the voltage-clamp condition, pilsicainide inhibited the transient outward current (Ito) that is more prominent in the atrium than in the ventricle in a concentration-dependent manner. However, in contrast to other class Ic agents, the inhibition to Ito by pilsicainide was observed only at much higher concentrations (IC50-300 uM) and did not affect the inactivation time-course of Ito. Moreover, the drug (10 uM) did not significantly affect the Ca2+, delayed rectifier K+, inward rectifying K+, acetylcholine-induced K+ or ATP-sensitive K+ currents. From these results pilsicainide could be differentiated as a pure Na+ channel blocker from other class Ic agents with diverse effects on membrane currents and should be recognized accordingly in clinical situations. A single oral dose of pilsicainide effectively restores normal sinus rhythm in patients with recent-onset atrial fibrillation and a healthy left ventricle. Long-term therapy with pilsicainide is successful in treating chronic atrial fibrillation