A novel in vivo experimental strategy, involving cell type-specific expression of a dominant-negative K+ channel pore-forming alpha subunit, was developed and exploited to probe the molecular identity of the cardiac transient outward K+ current (I(to)). A point mutation (W to F) was introduced at position 362 in the pore region of Kv4.2 to produce a nonconducting mutant (Kv4.2W362F) subunit. Coexpression of Kv4.2W362F with Kv4.2 (or Kv4.3) attenuates the wild-type currents, and the effect is subfamily specific; ie, Kv4.2W362F does not affect heterologously expressed Kv1.4 currents. With the use of the alpha-myosin heavy chain promoter to direct cardiac-specific expression, several lines of Kv4.2W362F transgenic mice were generated. Electrophysiological recordings reveal that I(to) is selectively eliminated in ventricular myocytes isolated from transgenic mice expressing Kv4.2W362F, thereby demonstrating directly that the Kv 4 subfamily underlies I(to) in the mammalian heart. Functional knockout of I(to) leads to marked increases in action potential durations in ventricular myocytes and to prolongation of the QT interval in surface ECG recordings. In addition, a novel rapidly activating and inactivating K+ current, which is not detectable in myocytes from nontransgenic littermates, is evident in Kv4.2W362F-expressing ventricular cells. Importantly, these results demonstrate that electrical remodeling occurs in the heart when the expression of endogenous K- channels is altered.