Tumor necrosis factor (TNF-) is a pro-inflammatory cytokine that has been implicated in the pathogenesis of heart failure. Prolongation of the action potential duration (APD) and down-regulation 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- modulates cardiac K⁺ channels. The present study analyzes the effect of TNF- on the transient outward K⁺ current (I_to) in rat ventricular myocytes, using the whole-cell patch-clamp technique. We found that TNF- is able to induce a significant reduction of Ito density, modifies its inactivation and down-regulates the Kv4.2 protein expression, while calcium current (I_CaL) density is not affected. We have also demonstrated that the reduction of I_to density induced by TNF- was prevented by the selective iNOS inhibitor 1400-W, the protein synthesis inhibitor cycloheximide, the antioxidant tocopherol and the superoxide dismutase mimetic MnTBAP. In addition, a reduced Ito density was recorded in ventricular myocytes exposed to peroxynitrite, supporting a possible participation of this oxidant in the effects of TNF- on I_to. We conclude that TNF- exposure, through iNOS induction and generation of oxidant species, promotes electrophysiological changes (decreased I_to and APD prolongation) in rat ventricular myocytes, providing new insights into how cytokines modulate K⁺ channels in the heart.