Aim This study was designed to identify the T-type calcium channel blocker Mibefradil as a cardioplegic agent in terms of its efficacy in immature cardioprotection. Methods By a Langendorff model, 32 hearts of 14- to 21-day-old New Zealand rabbits underwent 90 min of global hypothermic (15℃) ischemia protected with a different single dose of hypothermic (4℃) cardioplegia(group ST:St.Thomas solution; group SV:St.Thomas solution combined with 0.1 μmol/L Verapamil; group SM1:St.Thomas solution combined with 0.1 μmol/L Mibefradil;group SM2:St.Thomas solution combined with 1 μmol/L Mibefradil). The coronary flux, percent recovery of the cardiac function, LDH release, myocardium MDA, and myocardium ionized calcium were compared. Results There are no significant difference in coronary flux between the groups. Verapamil provided significantly the worst postreperfused percentage recoveryof the cardiac function (developed pressure: 0.70±0.23, dp/dt: 0.72±0.23, and -dp/dt: 0.67±0.25) than the other groups (p<0.05). There are no significant difference in the cardiac function between others groups. The higher concentration Mibefradil (1 μmol/L) showed a significant reduction of LDH release (6.6±2.2 IU/L vs 9.3±3.2 IU/L)in the coronary flow, and lower myocardium MDA (1.14±0.24 μmol/g vs 1.64±0.39 μmol/g) and ionized calcium (0.147±0.020 mg/g vs 0.184±0.021 mg/g) than group ST (p<0.05). Verapamil showed a highest LDH release (12.2±2.7 IU/L) in the coronary flow and myocardium ionized calcium (0.225±0.041 mg/g)(p< 0.05 ). Conclusion St.Thomas solution combined with Mibefradil provided better protection during ischemia-reperfusion in the immature rabbit heart than St.Thomas' solution, whereas St.Thomas' solution combined with Verapamil showed the worst protective effects in immature hearts