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Inhibition of cardiac late sodium current (late I Na ) is a strategy to suppress arrhythmias and sodium-dependent calcium overload associated with myocardial ischemia and heart failure. Current inhibitors of late I Na are unselective and can be proarrhythmic. This study introduces GS967 (6-[4-(trifluoromethoxy), a potent and selective inhibitor of late I Na , and demonstrates its effectiveness to suppress ventricular arrhythmias. The effects of GS967 on rabbit ventricular myocyte ion channel currents and action potentials were determined. Anti-arrhythmic actions of GS967 were characterized in ex vivo and in vivo rabbit models of reduced repolarization reserve and ischemia. GS967 inhibited Anemonia sulcata toxin II (ATX-II)-induced late I Na in ventricular myocytes and isolated hearts with IC 50 values of 0.13 and 0.21 mM, respectively. Reduction of peak I Na by GS967 was minimal at a holding potential of 2120 mV but increased at 280 mV. GS967 did not prolong action potential duration or the QRS interval. GS967 prevented and reversed proarrhythmic effects (afterdepolarizations and torsades de pointes) of the late I Na enhancer ATX-II and the I Kr inhibitor E-4031 in isolated ventricular myocytes and hearts. GS967 significantly attenuated the proarrhythmic effects of methoxamine1clofilium and suppressed ischemiainduced arrhythmias. GS967 was more potent and effective to reduce late I Na and arrhythmias than either flecainide or ranolazine. Results of all studies and assays of binding and activity of GS967 at numerous receptors, transporters, and enzymes indicated that GS967 selectively inhibited late I Na . In summary, GS967 selectively suppressed late I Na and prevented and/or reduced the incidence of experimentally induced arrhythmias in rabbit myocytes and hearts.