The effects of a new O-superfamily conotoxin, S03, on sodium current (INa), transient A-type potassium currents (IA), and delayed rectified potassium currents (IK), were examined in cultured rat hippocampal neurons using the whole-cell patch clamp technique. Addition of S03 caused a concentration-dependent, rapidly developing, and reversible inhibition of voltage-activated currents. The IC50 values for the blockage of INa, IA; and IK were calculated as 0.49, 33.9, and 7.6 μmol/l., respectively. The determined Hill coefficients were 1.7, 0.6, and 1.2, respectively. These results indicate that S03 can selectively inhibit neuronal sodium and potassium currents.