The effects of an oxidizing agent, chloramine-T, on the 4-aminopyridine-sensitive transient outward current (I T O ) were investigated in rabbit atrial myocytes by using patch-clamp techniques. Extracellular application of chloramine-T at 20 μM irreversibly slowed the time course of inactivation of the whole-cell I T O , and increased the peak by 19.3% (n=19) at +40 mV. At 100 μM, chloramine-T decreased the peak by 22.5% (n=9) of the control, and subsequently induced a glibenclamide-sensitive time-independent outward K + current. Under superfusion with dithiothreitol (3 mM), chloramine-T (100 μM) produced no change in I T O . The chloramine-T-induced slowing of I T O inactivation was partially reversed by subsequent application of 3 mM dithiothreitol. In single-channel recordings with the cell-attached patch configuration, chloramine-T (20 μM) increased the open probability of the I T O channel from 0.15 to 0.46 at a potential 100 mV positive to the resting potential, and the mean open lifetime from 5.1 ms to 7.0 ms (n=5). The unitary current amplitude was not affected. As a result, chloramine-T increased the ensemble current in amplitude and slowed its decay. These results indicated that: (1) inactivation of the native A-type channels of rabbit heart is susceptible to oxidation; and (2) oxidation of I T O channels may contribute to the genesis of arrhythmias.