This study tests the hypothesis that a decrease of the free energy of ATP hydrolysis (Δ G_ATP) below a threshold value will inhibit Na⁺-K⁺-ATPase (Na⁺pump) activity and result in an increase of intracellular Na⁺concentration ([Na⁺]_i) in the heart. Conditions were designed in which hearts were solely dependent on ATP derived from oxidative phosphorylation. The only substrate supplied was the fatty acid butyrate (Bu) at either low, 0.1 mM (LowBu), or high, 4 mM (HighBu), concentrations. Escalating work demand reduced the Δ G_ATPof the LowBu hearts.³¹P,²³Na, and⁸⁷Rb NMR spectroscopy measured high-energy phosphate metabolites, [Na⁺]_i, and Rb⁺uptake. Rb⁺uptake was used to estimate Na⁺pump activity. To measure [Na⁺]_iusing a shift reagent for cations, extracellular Ca²⁺was reduced to 0.85 mM, which eliminated work demand Δ G_ATPreductions. Increasing extracellular Na⁺([Formula: see text]) to 200 mM restored work demand Δ G_ATPreductions. In response to higher [Na⁺]_e, [Na⁺]_iincreased equally in LowBu and HighBu hearts to ∼8.6 mM, but Δ G_ATPdecreased only in LowBu hearts. At lowest work demand the LowBu heart Δ G_ATPwas –53 kJ/mol, Rb⁺uptake was similar to that of HighBu hearts, and [Na⁺]_iwas constant. At highest work demand the LowBu heart Δ G_ATPdecreased to –48 kJ/mol, the [Na⁺]_iincreased to 25 mM, and Rb⁺uptake was 56% of that in HighBu hearts. At the highest work demand the HighBu heart Δ G_ATPwas –54 kJ/mol and [Na⁺]_iincreased only ∼10%. We conclude that a Δ G_ATPbelow –50 kJ/mol limits the Na⁺pump and prevents maintenance of [Na⁺]_ihomeostasis.