This study tests the hypothesis that a decrease of the free energy of ATP hydrolysis (G_ATP) below a threshold value will inhibit Na⁺/K⁺-ATPase (Na⁺-pump) activity and result in an increase of intracellular sodium concentration ([Na⁺]_i) in the heart. Conditions were designed in which hearts were solely dependent upon ATP derived from oxidative phosphorylation. The only substrate supplied was the fatty acid, butyrate, (Bu) at either low, 0.1mM (LowBu), or high, 4 mM (HighBu), concentrations. Escalating work demand reduced the G_ATP of the LowBu hearts. ³¹P, ²³Na, and ⁸⁷Rb NMR spectroscopy measured high-energy phosphate metabolites,[Na⁺]_i and Rb⁺ uptake. Rb⁺ uptake was used to estimate Na⁺-pump activity. To measure [Na⁺]_i using a shift reagent for cations, extracellular Ca²⁺ was reduced to 0.85 mM, which eliminated work demand G_ATP reductions. Increasing extracellular Na⁺ (Na⁺_e) to 200 mM restored work demand G_ATP reductions. In response to higher [Na⁺]_e, [Na⁺]_i increased equally in LowBu and HighBu hearts to ~8.6 mM but G_ATP decreased only in LowBu hearts. At lowest work demand the LowBu hearts G_ATP was -53 kJ/mol, Rb⁺ uptake was similar to that of HighBu hearts, and [Na⁺]_i was constant. At highest work demand the LowBu hearts G_ATP decreased to –48 kJ/mol, the [Na⁺]_i increased to 25 mM, and Rb⁺ uptake was 56% of that in HighBu hearts. At the highest work demand the HighBu hearts G_ATP was –54 kJ/mol and [Na⁺]_i increased only ~10%. We conclude that a G_ATP below –50 kJ/mol limits the Na⁺-pump and prevents maintenance of [Na⁺]_i homeostasis.