Objective To determine the myocardial protective effect of crystalloid cardioplegic solution at different calcium concentration on immature myocardium.Methods Isolated perfused neonatal rabbit hearts from three groups, arrested by intermittent infusion of St. Thomas Ⅱ cardioplegic solution with different concentration of calcium (in each group, only calcium concentration of cardioplegic solution was modified, Ⅰ.[Ca2+]0.6 mmol/L; Ⅱ.[Ca2+]1.2 mmol/L; Ⅲ.[Ca2+]2.4 mmol/L), were kept ischemic globally at 20℃ for 90 minutes and then followed by 30 minutes of reperfusion in Langendorff mode. Results Although the recovery of LVDP, +dp/dtmax at calcium content of 2.4 mmol/L after 10 minutes of reperfusion was significantly higher than those at 0.6 and 1.2 mmol/L calcium (P<0.05, P<0.01, respectively). The declined tendency of left ventricular hemodynamics after 20 minutes of reperfusion in this group was detected. By the end of reperfusion, the left ventricular functional recovery at 2.4 mmol/L calcium did not differ from those at 1.2 and 0.6 mmol/L calcium. Conversely, postischemic left ventricular functions at 0.6 and 1.2 mmol/L calcium were gradually improved during 30 minutes of reperfusion. In 2.4 mmol/L calcium group, the Ca2+-ATPase activity significantly increased (P<0.01, P<0.001) whereas myocardial ATP content was lower when compared with 1.2 mmol/L (P<0.001) and 0.6 mmol/L calcium groups.Conclusions Our research demonstrated that there were no statistical differences with respect to hemodynamic recovery in three groups after 30 minutes of reperfusion although left ventricular functional recovery at 2.4 mmol/L calcium accelerated early after reperfusion. In addition, with 2.4 mmol/L calcium, myocardial ATP content was decreased significantly. We conclude that, from the point of view of myocardial energy metabolism, St. Thomas Ⅱ cardioplegic solution at high concentration of calcium can not provide immature myocardium with optimal myocardial protection while with 1.2 mmol/L calcium, however, better high-energy store can be preserved.