It was recently reported that the slow transient outward K(+) current, I(to, s), that is evident in mouse left ventricular septal cells is eliminated in mice with a targeted deletion of the Kv1.4 gene (Kv1.4(-/-)). The rapidly inactivating transient outward K(+) current, I(to, f), in contrast, is selectively eliminated in ventricular myocytes isolated from transgenic mice expressing a dominant-negative Kv4 alpha subunit, Kv4.2W362F. Expression of Kv4. 2W362F results in marked prolongation of action potentials and QT intervals. In addition, a slow transient outward K(+) current, that is similar to I(to,s) in wild-type mouse left ventricular septal cells, is evident in all Kv4.2W362F-expressing (left and right) ventricular cells. To test directly the hypothesis that upregulation of Kv1.4 alpha subunit underlies the appearance of this slow transient outward K(+) current in Kv4.2W362F-expressing ventricular cells and to explore the functional consequences of elimination of I(to,f) and I(to,s), mice expressing Kv4.2W362F in the Kv1.4(-/-) background (Kv4.2W362FxKv1.4(-/-)) were generated. Histological and echocardiographic studies revealed no evidence of structural abnormalities or contractile dysfunction in Kv4.2W362FxKv1.4(-/-) mouse hearts. Electrophysiological recordings from the majority (approximately 80%) of cells isolated from the right ventricle and left ventricular apex of Kv4.2W362FxKv1.4(-/-) animals demonstrated that both I(to, f) and I(to,s) are eliminated; action potentials are prolonged significantly; and, in some cells, early afterdepolarizations were observed. In addition, in vivo telemetric ECG recordings from Kv4.2W362FxKv1.4(-/-) animals revealed marked QT prolongation, atrioventricular block, and ventricular tachycardia. These observations demonstrate that upregulation of Kv1.4 contributes to the electrical remodeling evident in the ventricles of Kv4.2W362F-expressing mice and that elimination of both I(to,f) and I(to,s) has dramatic functional consequences.