Background— Dysregulation of voltage-gated cardiac Na ⁺ channels (Na _V 1.5) by inherited mutations, disease-linked remodeling, and drugs causes arrhythmias. The molecular mechanisms whereby the Na _V 1.5 voltage-sensing domains (VSDs) are perturbed to pathologically or therapeutically modulate Na ⁺ current ( I _Na ) have not been specified. Our aim was to correlate I _Na kinetics with conformational changes within the 4 (DI–DIV) VSDs to define molecular mechanisms of Na _V 1.5 modulation. Method and Results— Four Na _V 1.5 constructs were created to track the voltage-dependent kinetics of conformational changes within each VSD, using voltage-clamp fluorometry. Each VSD displayed unique kinetics, consistent with distinct roles in determining I _Na . In particular, DIII-VSD deactivation kinetics were modulated by depolarizing pulses with durations in the intermediate time domain that modulates late I _Na . We then used the DII-VSD construct to probe the molecular pathology of 2 Brugada syndrome mutations (A735V and G752R). A735V shifted DII-VSD voltage dependence to depolarized potentials, whereas G752R significantly slowed DII-VSD kinetics. Both mutations slowed I _Na activation, although DII-VSD activation occurred at higher potentials (A735V) or at later times (G752R) than ionic current activation, indicating that the DII-VSD allosterically regulates the rate of I _Na activation and myocyte excitability. Conclusions— Our results reveal novel mechanisms whereby the Na _V 1.5 VSDs regulate channel activation and inactivation. The ability to distinguish distinct molecular mechanisms of proximal Brugada syndrome mutations demonstrates the potential of these methods to reveal how inherited mutations, post-translational modifications, and antiarrhythmic drugs alter Na _V 1.5 at the molecular level.