The effect of oxidants on voltage dependent K⁺ currents was examined in mouse colonic smooth muscle cells. Exposure to either chloramine T (Ch-T), an agent known to oxidize both cysteine and methionine residues, or the colon-specific oxidant monochloramine (NH₂Cl) completely suppressed the transient outward K⁺ current (I_TO) while simultaneously enhancing the sustained delayed rectifier K⁺ current (I_DR). In contrast, the cysteine-specific oxidants hydrogen peroxide (H₂O₂) and 5,5[[rad]]-dithio-bis(2-nitrobenzoic acid) (DTNB) exhibited partial and slow suppression of I_TO by inducing a shift in channel availability of -18 mV without affecting I_DR. After enhancement by NH₂Cl or Ch-T, I_DR was inhibited by 10 mM TEA but not other K⁺ channel blockers, suggesting that it represented activation of the resting I_DR and not a separate K+ conductance. Extracellular dithiothreitol (DTT) partially reversed the effect of H₂O₂ and DTNB on I_TO but not the actions of NH₂Cl and Ch-T on either I_DR or I_TO. Dialysis of myocytes with glutathione (GSH; 5 mM) or DTT (5 mM) prevented suppression of I_TO by H₂O₂ and DTNB but did not alter the effects of NH₂Cl or Ch-T on either I_SUS or I_TO. Ch-T and NH₂Cl completely blocked I_TO generated by mK_v 4.1, 4.2, and 4.3 in xenopus oocytes, an effect not reversible by intracellular DTT. In contrast, intracellular DTT reversed the effect of H₂O₂ and DTNB on the cloned channels. These results suggest that I_TO is suppressed via modification of both methionine and cysteine residues whereas enhancement of I_DR may result from methionine oxidation alone.