The cellular bioenergetic responses of isolated perfused working rat hearts to alterations in hemodynamic function caused by acute exposure to elevated levels of extracellular magnesium ions ([Mg2+]o) were examined using 31P nuclear magnetic resonance (31P NMR) spectroscopy. Results showed that in hearts working against 90 cm H2O afterload, an increase in [Mg2+]o from 1.2 to 4. mM reduced the heart rate by 35%, while coronary flow was increased by 38%. Unexpectedly, despite the pronounced bradycardia, the rate-pressure product was reduced only slightly (from 2.36×104 to 2.08×104 mm Hg/min) due to a significant increase (36%) in systolic pressure. In addition, cardiac output actually increased by 23%, owing to a > 100% increase in stroke volume, indicating that the performance of the heart was improved and suggesting that the efficiency of the heart was improved as well. In a separate series of experiments, 31P NMR measurements performed on hearts perfused in the Langendorff mode revealed that elevated level of [Mg2+]o increase phosphocreatine (PCr) levels by 23% (from 9.2 to 11.3 mM), while Pi levels declined by a corresponding amount. Perfusion of hearts in the working mode with elevated [Mg2+]o was also observed to increase PCr levels from 6.3 to 9.0 mM, while ATP levels declined by 17%. Measurement of the chemical shift difference between Pi and PCr and that between the α and β phosphate resonances of ATP were used to determine intracellular pH and the cytosolic levels of free Mg2+ ([Mg2+]i), respectively. These results showed that acute exposure of hearts, perfused in either the working or Langendorff mode, to increased levels of [Mg2+]o increased intracellular pH by 0.12–0.13 units, while free Mg2+ nearly doubled to a level of 1.1–1.2 mM. The latter observation may suggest that acute variations in the level of [Mg2+]o can influence a multitude of cellular processes requiring Mg2+ as an essential cofactor. Using the above data and assuming equilibrium of the creatine kinase reaction, the levels of ADP, cytosolic phosphorylation potential ([ATP]/[ADP][Pi]) and free energy change from ATP hydrolysis (-δG/δE) were also calculated. Results obtained illustrate that in the presence of elevated [Mg2+]o ADP levels declined by 33–48%, the cytosolic phosphorylation potential increased from 41 to 112 mM-1 and -δG/δE increased from 56.7 to 59.3 kJ/mol. These changes are not completely accountable by the known bradycardia and vasodilatory effects of elevated [Mg2+]o and strongly argue for a direct action of [Mg2+]o on the myocyte as well. The observed increase in cardiac output and stroke volume could be suggestive of a positive inotropic–like effect of elevated [Mg2+]o, and concomitant cellular energetic changes are unlike those of any previously described agent. Elevated [Mg2+]o unloads the heart, reduces O2 consumption and improves cardiac efficiency while improving coronary flow and increasing intracellular stores of high–energy phosphates. These data when coupled with other findings form a rational basis for employing Mg2+ therapy in cardiac diseases and suggest that such measurements performed on the intact human patient should prove calculable in clinical diagnosis and treatment of cardiac diseases.