Nitric oxide (NO) reacts rapidly with superoxide anion and produces speroxynitrite (ONOO^－), which is a highly reactive free radical that has been shown to mediate much of the toxicity of NO. ONOO^－ has also been implicated as playing a role in the pathology of various brain disorders. The aim of the study was to investigate the actions of ONOO^－ on voltage-gated sodium currents and its effect on membrane excitability in hippocampal CA1 neurons. SIN-1, which leads to the simultaneous generation of superoxide anion and NO, and then forms the highly reactive species ONOO^－, induced a dose-dependent inhibition in amplitudes of transient potassium currents (I_Na). SIN-1(500 μmol/L) delayed the recovery of I_Na from inactivity, but did not have a marked effect on the activation and inactivation parameters of I_Na. However, co-treamtent with SIN-1 and Urate (100 mmol/L), an ONOO^－ scavenger, had no effect on I_Na currents. SIN-1 (500 μmol/L) led to the decrease in the amplitude and firing rate of action potential. Pretreatment with adenylate cyclase inhibitor MDL-12, 330A (25 μmol/L) and NEM (50 μmol/L) did not block the effects of ONOO^－. On the other hand, the responses to ONOO^－ were inhibited by guanylate cyclase inhibitor ODQ (10 μmol/L). The effects of ONOO^－ on hippocampal neurons were mediated through activation of cGMP-I_Na-AP (action potential) signaling cascades and independent from PKA and S-nitrosylation pathway, which maybe one of underlying mechanisms likely related to neurotoxicity of ONOO^－.