Atrial fibrillation (AF), the most prevalent clinical arrhythmia, is associated with increased mortality and morbidity. Voltage-gated Na+ channel blockers are highly effective in cardioversion and maintenance of sinus rhythm in patients with early onset AF but carry a significant risk of lethal ventricular tachyarrhythmias <!–[endif]—->(Camm, 2012; Lafuente-Lafuente et al., 2015). It has been suggested that atrial-ventricular differences in voltage-gated Na+ currents may be exploited for atrial-selective antiarrhythmic drug action for the suppression of AF without risk of ventricular side-effects (Hancox et al., 2016). Eleclazine (GS-6615) is a putative anti-arrhythmic drug, selective for the late component of the Na+ current, that has been shown to be safe and well-tolerated in patients and has properties similar to the prototypical atrial-selective Na+ channel blocker, ranolazine <!–[endif]—->(Caves et al., 2017; El-Bizri et al., 2018). The present study investigated atrial-ventricular differences in voltage-gated Na+ currents and their inhibition by eleclazine. Procedures were approved by local ethics committee and performed in accordance with UK legislation. Atrial and ventricular myocytes were isolated from adult male Wistar rat hearts. The fast and late components of voltage-gated Na+ currents (respectively, INa & INaL) were recorded at room temperature (~22 °C) using whole-cell patch clamp recording techniques. INa was recorded using symmetrical external and internal [Na+] (5 mM). INaL was recorded in separate experiments using an external [Na+] of 70 mM (internal [Na+] = 5 mM). INaL was activated by superfusion with the sea anemone toxin, ATX-II (3 nM). The half-maximal voltages of activation (-47.0±1.4 mV, n=10) and inactivation (-94.3±0.5 mV, n=11) of atrial INa were more negative than the corresponding values for ventricular INa (respectively, -41.5±1.3 mV, n=10, P<0.05 and -87.2±0.4 mV, n=11, P<0.0001; Student’s t-test). There was no difference between cell types in the maximally activated Na+ conductance density. There was no difference between atrial and ventricular myocytes in the eleclazine inhibition of INaL (IC50s ~200 nM for both) in the presence of ATX-II. Eleclazine (10 μM) inhibited INa in atrial and ventricular myocytes in a use-dependent manner consistent with preferential activated state block. There was no difference between atrial and ventricular myocytes in the use-dependent inhibition of INa. However, eleclazine produced an instantaneous inhibition of INa in atrial but not in ventricular myocytes. In summary, differences exist between rat atrial and ventricular myocytes in the biophysical properties of INa. The more negative voltage-dependence of INa activation and inactivation underlies the instantaneous inhibition by eleclazine in atrial myocytes. Eleclazine warrants further investigation as an atrial-selective anti-arrhythmic drug.<!–![endif]—-><!–![endif]—->