Oxidative stress and the production of reactive oxygen species are known to play a major role in neuronal cell damage, but the exact mechanisms responsible for neuronal injury and death remain uncertain. In the present study, we examined the effects of oxidative stress on spontaneous spike activity and depolarizing outward potassium current by exposing the Retzius neurons of the leech to cumene hydroperoxide (CHP) and hydrogen peroxide (H(2)O(2)), the oxidants commonly used to examine oxidative mechanisms mediating cell death. We observed that relatively low concentrations of CHP (0.25, 1, and 1.5 mM) led to a marked prolongation of spontaneous repetitive activity. The prolonged action potentials showed an initial, spike-like depolarization followed by a plateau phase. In contrast, H(2)O(2) at the same and much higher concentrations (0.25 to 5 mM) did not significantly change the duration of spontaneous spike potentials of leech Retzius nerve cells (LRNCs). In the voltage clamp experiments, calcium-activated outward potassium currents, needed for the repolarization of the action potential, were suppressed with CHP, but not with H(2)O(2). The present findings indicate that CHP is a more potent oxidant and neurotoxin than H(2)O(2) and that the effect of CHP on the electrophysiological properties of LRNCs may be due to the inhibition of the potassium channels.