Hypothermic cardioplegia is a well established method to optimize myocardial preservation during ischemic arrest, and it has been demonstrated that oxygenation of crystalloid cardioplegic solutions markedly enhances myocardial protection, The addition of a small amount of red blood cells to a crystalloid cardioplegic solutions improves capillary perfusion. Considering these results, we changed our cardioplegic solution from cold oxygenated crystalloid[Group 2] to cold oxygenated diluted blood[Group 1]. In this investigation, we examined the effects of two hypothermic potassium cardioplegic solutions on myocardial preservation in 50 patients[30 of Group 1 and 20 of Group 2] of child age group. Factors considered preoperatively included age, sex, body weight, preoperative diagnosis, and they showed no statistical differences, Intraoperative factors considered included duration of cardiopulmonary bypass, duration of aortic occlusion, operative mortality, which also revealed no statistically significant differences, We measured the serum levels of GOT[glutamate oxaloacetate transaminase] and CPK [creatine phosphokinase] during the first two days postoperatively, which, in both groups, showed significantly higher values until postoperative 1 day, and decreasing tendancy thereafter, however we failed to find any significant difference between two groups regarding the serum levels of those enzymes each day. Time for extubation and use of inotropics also revealed no significant differences. Defibrillation was needed less in Group 1 than in Group 2[p<0.05], and one case of supraventricular tachyarrhythmia occured in Group l. We conclude that cold oxygenated diluted blood cardioplegia provides no less preservation than does an oxygenated crystalloid cardioplegic solution in child age group.