Characteristics of voltage -dependent sodium current recorded from adult rat muscle fibers in loose patch mode were rapidly altered following nearby impalement with a microelectrode. Hyperpolarized shifts in the voltage dependence of activation and fast inactivation occurred within minutes. In addition, the amplitude of the maximal sodium current decreased within 30 minutes of impalement. Impalement triggered a sustained elevation of intracellular Ca²⁺. However, buffering Ca²⁺ by loading fibers with AM-BAPTA did not affect the hyperpolarized shifts in activation and inactivation, although it did prevent the reduction in current amplitude. Surprisingly, the rise in intracellular Ca²⁺ occurred even in the absence of extracellular Ca²⁺. This result indicated that the injury-induced Ca²⁺ increased came from an intracellular source, but it was not blocked by an inhibitor of release from the sarcoplasmic reticulum, which suggested involvement of mitochondria. Massive Ca²⁺ release from mitochondria triggered by CCCP was sufficient to cause a reduction in sodium current amplitude, but had little effect of the voltage dependence of activation and fast inactivation. Our data suggest the effects of muscle injury can be separated into a Ca²⁺-dependent reduction in amplitude and a largely Ca²⁺- independent shift in activation and fast inactivation. Together, the impalement-induced changes in sodium current reduce the number of sodium channels available to open at the resting potential and may limit further depolarization, and thus promote survival of muscle fibers following injury.