Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine that has been implicated in the pathogenesis of heart failure. Prolongation of the action potential duration and downregulation of several K(+) currents might participate in the genesis of arrhythmias associated with chronic heart failure. Little information is available related to the mechanism by which TNF-alpha modulates cardiac K(+) channels. The present study analyzes the effect of TNF-alpha on the transient outward K(+) current (I(to)) in rat ventricular myocytes, using the whole cell patch-clamp technique. We found that TNF-alpha is able to induce a significant reduction of I(to) density, modifies its inactivation, and downregulates the Kv4.2 protein expression, while calcium current density is not affected. We have also demonstrated that the reduction of I(to) density induced by TNF-alpha was prevented by the selective inducible nitric oxide synthase (iNOS) inhibitor 1400-W, the protein synthesis inhibitor cycloheximide, the antioxidant tocopherol, and the superoxide dismutase mimetic manganese(III) tetrakis (4-benzoic acid) porphyrin. In addition, a reduced I(to) density was recorded in ventricular myocytes exposed to peroxynitrite, supporting a possible participation of this oxidant in the effects of TNF-alpha on I(to). We conclude that TNF-alpha exposure, through iNOS induction and generation of oxidant species, promotes electrophysiological changes (decreased I(to) and action potential duration prolongation) in rat ventricular myocytes, providing new insights into how cytokines modulate K(+) channels in the heart.