[Na+]i is found to be elevated in several models of heart failure. This is believed to favor Ca2+ extrusion from mitochondria via the mNCX and therefore reduce mitochondrial Ca2+ accumulation. As several dehydrogenases of the mitochondrial Krebs cycle are stimulated by Ca2+, the defects in cellular sodium homeostasis may induce mitochondrial oxidative stress. Previously we have estimated that resting [Na+]i is significantly elevated in ventricular myocytes isolated from dystrophic (mdx) mice compared to those from wild-type (WT) mice (24.2 ± 3.1 mM, n=13 vs. 14.0 ± 1.7 mM, n=9). Moreover, the mitochondrial matrix was significantly oxidized at resting conditions (35±3.1 %, n=27 vs. 53±5.2 %, of NADH reduction n=10, in mdx and WT respectively). Here maximal oxidation of NADH by FCCP/oligomycin was taken as 0% reduced NADH and maximal reduction by rotenone and β-hydroxy-butyrate was defined as 100% reduced NADH. We suggested that beat-to-beat mechanical activity in dystrophic heart might lead to greater [Na+]i accumulation and oxidation of the mitochondrial matrix. Here we investigated whether elevated [Na+]i affects mitochondrial energetics during excitation-contraction coupling in dystrophic myocytes. We measured NADH autofluorescence in WT and mdx cells under control conditions and during 4Hz stimulation in the presence of isoproterenol. An abrupt increase in workload resulted in significant oxidation of NADH both in mdx and WT cardiomyocytes. However, the extent of oxidation was significantly greater in dystrophic cells (50±9 %, n=10 vs 15±6 %, n=11 decrease in NADH, respectively). When workload was discontinued, the mitochondrial matrix quickly returned back to its base line levels in WT but not in mdx cells. These results support the notion that the elevated [Na+]i contributes to the impaired energetics in dystrophic cardiomyocytes and therefore contributes to the development of dystrophic cardiomyopathy.