In this study,²³Na- and ³¹P- nuclear magnetic resonance (NMR) spectra were examined in perfused rat hearts harvested 1, 2, 4, and 24 h after 40% total body surface area burn trauma and lactated Ringer resuscitation, 4 ml · kg⁻¹ · %⁻¹ burn.²³Na-NMR spectroscopy monitored myocardial intracellular Na⁺ using the paramagnetic shift reagent thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylenephosphonic acid). Left ventricular function, cardiac high-energy phosphates (ATP/PCr), and myocyte intracellular pH were studied by using³¹P NMR spectroscopy to examine the hypothesis that burn-mediated acidification of cardiomyocytes contributes to subsequent Na⁺ accumulation by this cell population. Intracellular Na⁺ accumulation was confirmed by sodium-binding benzofuran isophthalate loading and fluorescence spectroscopy in cardiomyocytes isolated 1, 2, 4, 8, 12, 18, and 24 h postburn. This myocyte Na⁺ accumulation as early as 2 h postburn occurred despite no changes in cardiac ATP/PCr and intracellular pH. Left ventricular function progressively decreased after burn trauma. Cardiomyocyte Na⁺ accumulation paralleled cardiac contractile dysfunction, suggesting that myocardial Na⁺overload contributes, in part, to the progressive postburn decrease in ventricular performance.