Intracellular Ca²⁺ ([Ca²⁺]_i) can trigger dual-mode regulation of the voltage gated cardiac sodium channel (Na_V1.5). The channel components of the Ca²⁺ regulatory system are the calmodulin (CaM)-binding IQ motif and the Ca²⁺ sensing EF hand–like (EFL) motif in the carboxyl terminus of the channel. Mutations in either motif have been associated with arrhythmogenic changes in expressed Na_V1.5 currents. Increases in [Ca²⁺]_i shift the steady-state inactivation of Na_V1.5 in the depolarizing direction and slow entry into inactivated states. Mutation of the EFL (Na_V1.5_4X) shifts inactivation in the hyperpolarizing direction compared with the wild-type channel and eliminates the Ca²⁺ sensitivity of inactivation gating. Modulation of the steady-state availability of Na_V1.5 by [Ca²⁺]_i is more pronounced after the truncation of the carboxyl terminus proximal to the IQ motif (Na_V1.5₁₈₈₅), which retains the EFL. Mutating the EFL (Na_V1.5_4X) unmasks CaM-mediated regulation of the kinetics and voltage dependence of inactivation. This latent CaM modulation of inactivation is eliminated by mutation of the IQ motif (Na_V1.5_4X-IQ/AA). The LQT3 EFL mutant channel Na_V1.5_D1790G exhibits Ca²⁺ insensitivity and unmasking of CaM regulation of inactivation gating. The enhanced effect of CaM on Na_V1.5_4X gating is associated with significantly greater fluorescence resonance energy transfer between enhanced cyan fluorescent protein–CaM and Na_V1.5_4X channels than is observed with wild-type Na_V1.5. Unlike other isoforms of the Na channel, the IQ-CaM interaction in the carboxyl terminus of Na_V1.5 is latent under physiological conditions but may become manifest in the presence of disease causing mutations in the CT of Na_V1.5 (particularly in the EFL), contributing to the production of potentially lethal ventricular arrhythmias.