It has been previously reported that the transgenic mouse expressing the dominant negative mutant of the neuron restrictive silencing factor (dnNRSF) in the heart died from lethal arrhythmia, so the present study aimed to clarify the electrophysiological alteration of the ventricular myocyte isolated from the dnNRSF mouse.The action potential (AP) and membrane currents were recorded using the whole-cell patch-clamp method. Intracellular Ca(2+) was measured with Indo-1AM. The AP of dnNRSF myocytes exhibited reduction of resting membrane potential, prolongation of AP duration, and frequent early afterdepolarization (EAD). The EAD was completely inhibited by SEA0400, a specific blocker of the Na(+)-Ca(2+) exchanger (NCX). The most notable alteration of membrane current was a reduction in the inward rectifier K(+) current (I(K1)) density. In addition to re-expression of fetal type cardiac ion channels, a Na(+)-permeable, late inward current was observed in a small population of dnNRSF myocytes. The diastolic intracellular Ca(2+) concentration was also raised in dnNRSF myocytes, and spontaneous Ca(2+) oscillation was induced by β-adrenergic stimulation.In dnNRSF myocytes, the "repolarization reserve" of the AP was significantly reduced by specific alterations in membrane currents. Under these conditions, the amplitude of EAD generated by the inward NCX current might be enlarged, thereby increasing the cells' vulnerability to ventricular arrhythmia.