SUMMARY1. The effects of externally applied chloramine-T on the excitability of single toad myelinated nerve fibres were studied. Chloramine-T is a mild oxidant which reacts specifically with the cysteine and methionine residues of proteins.2. Chloramine-T prolongs the action potential of a single myelinated fibre by more than 1000-fold. This effect is concentration-and time-dependent; higher concentrations and longer incubation times increase prolongation.3. Under voltage-clamp conditions, sodium channel inactivation is markedly inhibited by chloramine-T while sodium channel activation remains normal.4. Prolonged depolarization of the membrane leads to a maintained sodium current. The maintained sodium currents show activation kinetics, dependence on membrane potential, and reversal potentials which are similar to those of normal, inactivating sodium currents in untreated fibres. Both the maintained and the peak sodium currents are equally inhibited by tetrodotoxin.5. After partial removal ofsodium inactivation by briefexposures to chloramine-T, the voltage dependence of the steady-state sodium current inactivation (h.) is shifted in the depolarized direction by about 20 mV, even after correction for the noninactivating component contributed by the maintained current. 6. The phenomena described here imply that cysteine or methionine residues are critical for the sodium channel inactivation processes. The two different modifications of inactivation, its removal shown by the maintained current, and the shift in the voltage-dependence of the remaining inactivatable channels, reveal that at least two separate residues are modified by chloramine-T.