Although MS-551 is classified as a class III antiarrhythmic agent (K+ channel blocker), its effect on the Na+ channel has not been fully characterized. We investigated the effect of MS-551 on the Na+ current (INa) in isolated guinea pig ventricular myocytes. MS-551 blocked INa in a concentration-dependent manner at a holding potential of −140 mV. The concentration-response curve revealed that the median inhibitory concentration (IC50) for the block of resting channel was 292 ± 20 μM with a Hill coefficient of 1 (n = 11). Although MS-551, 300 μM, did not show a use-dependent block, it shifted the steady-state inactivation curve in a hyperpolarizing direction by 6.3 ± 0.8 mV and delayed the recovery process from long depolarization. This delay was considered to be related to the drug unbinding and was expressed by a triple exponential function. The slowest component had a time constant of 409 ± 35 ms, and the proportion of the amplitude of this component to the total current amplitude was 14 ± 3% (n = 6). The IC50 for the inactivated Na+ channel was thus estimated to be 169 μM at maximum. These results suggest that MS-551 has a low affinity for both the resting and inactivated Na+ channel.