Oxidation of amino acid residues causes noticeable changes in gating of many ion channels. We found that P/C-type inactivation of Shaker potassium channels expressed in Xenopus oocytes is irreversibly accelerated by patch excision and that this effect was mimicked by application of the oxidant H2O2, which is normally produced in cells by the dismutase action on the superoxide anion. The inactivation time course was also accelerated by high concentration of O2. Substitution of a methionine residue located in the P- segment of the channel with a leucine largely eliminated the channel's sensitivity to patch excision, H2O2, and high O2. The results demonstrate that oxidation of methionine is an important regulator of P/C-type inactivation and that it may play a role in mediating the cellular responses to hypoxia/hyperoxia.