The protective effect of preoperative intravenous infusion of high doses of insulin (2U/100gm) was studied in isolated rat heart models, using modified Langendorff apparatus. Fifteen rat hearts were equally divided into three groups: Group Ⅰ(control group) received glucose and potassium within 60 min. prior to the onset of operation. Group Ⅱ (insulin group) was administered with glucose, insulin and potassium(GIK)within 60 min. before operation, whereas Group Ⅲ(insulin cardioplegia group)was treated with the same amount of GIK Preoperatively as well as an addition of insulin (100U/L) to the cardioplegia. All the rat hearts were subjected to a 30-min. period of ischemia. Coronary effluent, lactic dehydrogenase (LDH) of coronary effluent, myocardial water content, histochemistry and ultrastructure of myocardium were studied. After reperfusion, the amount of coronary effluent was 2.84±0.79 ml/min/100gm in Group Ⅱ, 1.32±0.45 ml/min/100gm in Group Ⅰ and 1.72±1.03 ml/min/100gm in Group Ⅲ, respectively, (Group Ⅱ vs Group Ⅰ or Ⅲ p<0.05), LDH was significantly lower in Group Ⅱ (4.48±1.24 U/L) than in Group Ⅰ (25.16±8.96 U/L) (P<0.01) and in Group Ⅲ (17.8±12.2 U/L) (P<0.05), but no significant difference could be seen between Group Ⅰ and Group Ⅲ. Myocardial water content was 71.5±1.21% in Group Ⅱ and 78.57±3.14% in Group Ⅰ (p<0. 01), whereas it was 75.42±3.28% in Group Ⅲ(vs Group Ⅰ and Ⅱ p>0.05).Histochemical succinodehydrogenase (SDH) reaction was graded 0 to 1 in Group Ⅱ and 2 to 3 in Group Ⅰ and Ⅲ (p<0.01). Group Ⅱ showed nearly normal myocardial ultrastructure with increased glycogen stores, while Groups Ⅰ and Ⅲ revealed local myofibrillary disorganization. disrupted mitochondria and Pronounced depletion of glycogen storages. These results prove that the Protective effect of preoperative application of high doses of insulin together with glucose and potassium on isolated rat hearts is definitely superior to the use of glucose and potassium alone. These findings also suggest that addition of insulin to cardioplegia is ineffective and even harmful to the ischemic myocardium. Folicie heart hearts are equally divided into three groups: Group I (control group Within 60 min. prior to the onset of operation. Group II (insulin group) was administered with glucose, insulin and potassium(GIK)within 60 min. before operation, but Group III(insulin cardioplegia group) was treated with the same amount of Coronary effluent, lactic dehydrogenase (LDH) of coronary effluent, myocardial water content, GIK Preoperatively as well as an addition of insulin (100U/L) to the cardioplegia. All the rat hearts were issues to a 30-min. period of ischemia. Histochemistry and ultrastructure of myocardium were studied. After reperfusion, the amount of coronary effluent was 2.84±0.79 ml/min/100gm in Group II, 1.32±0.45 ml/min/100gm in Group I and 1.72±1.03 ml/min/100gm i n Group III, respectively, (Group II vs Group I or III p<0.05), LDH was significantly lower in Group II (4.48±1.24 U/L) than in Group I (25.16±8.96 U/L) (P<0.01 ) and in Group III (17.8±12.2 U/L) (P<0.05), but no significant difference could be seen between Group I and Group III. Myocardial water content was 71.5±1.21% in Group II and 78.57±3.14% in Group I (p<0. 01), while it was 75.42±3.28% in Group III (vs Group I and II p>0.05). Histochemical succinodehydrogenase (SDH) reaction was graded 0 to 1 in Group II and 2 to 3 in Group I showed nearly normal myocardial ultrastructure with elevated glycogen stores, while Groups I and III revealed local myofibrillary disorganization. disrupted mitochondria and Pronounced depletion of glycogen storages. These results prove that the protective effect of preoperative Application of high doses of insulin together with glucose and potassium on isolated rat hearts is definitely superior to the us e of glucose and potassium alone. These previous searchesEst that addition of insulin to cardioplegia is ineffective and even harmful to the ischemic myocardium.