Aim To evalute the protective effects of St. Thomas NO.2 cardioplegia and blood cardioplegia in protecting myocardium of immature rabbit hearts from ischemic reperfusion injury. Methods On modified Langendorff apparatus, 12 isolated hearts of 14～21 d immature rabbit were divided in two groups randomly: St.Thomas NO.2 cardioplgeia and blood cardiplegia and subjected to 2 h of ischemia and 1 h of reperfusion. The preischemia and postischemia myocardial function were assessed by the percentage recovery of left ventricle function, myocardial water content, and the leakage of myocardial enzyme. Results Postischemic recovery of left ventricle function were significantly better in blood cardiplegia group than in St.Thomas NO.2 cardioplgeia group, and St. Thomas NO.2 cardioplgeia group had higher percentage of leakage of myocardial enzyme. But there were no differences in myocardial water content. Conclusion The blood cardioplegia can provide better myocardial protection for immature rabbit hearts than the St. Thomas NO.2 cardiolplegia.