Objective: To assess the protective mechanism of cold cardioplegia in ischemic hearts by observating effects of different protective methods on ischemic myocardial intracellular energy metabolism and ion homeostasis. Methods: Isolated rat hearts were perfused in a Langendorff preparation with a modified Krebs-Henseleit medium with 3.5 mmol/L Tm(DOTP)5-, a sodium shift agent. Experimental groups underwent 60 min ischemia(groupⅠ) either after a 5 min perfusion of hyperkalemic cardioplegia(groupⅡ) or at 15℃(group Ⅲ), or with both interventions(group Ⅳ), followed by 30 min of reperfusion. Interleaved 31P and 23Na NMR spectra were continuously monitored to measure ATP, Pcr,Phi,Nai and Pi during ischemia and reperfusion. Left ventricular function was monitored with an intraventricular balloon simultaneously. Results: The myocardium showed intracellular acidosis and sodium accumulation severely in group Ⅰ, ATP and Pcr of which being not detected after 15 min of ischemia. After 30 min of ischemia, the changes of group Ⅱ were similar to those of group Ⅰ. The changes of group Ⅲ were more slight compared to groupⅠ and Ⅱ during ischemia. ATP and Nai of group Ⅳ were maintained. The left ventricular function was preserved in group Ⅲ and Ⅳ after reperfusion. Conclusion: Hypothermia as the sole intervention was more effective than hyperkalemic on ischemic myocardial metabolism and function. The combination of hypothermia and hyperkalemia could prevent ischemic myocardium from intracellular acidosis and eliminate sodium accumulation effectively, so it plays a greater protective role in ischemic myocardial function.