Hyperglycemia has been noted to occur in pediatric patients undergoing cardiac surgery with hypothermia. Because even moderate hyperglycemia during cerebral ischemia may predispose patients to an increased risk of neurologic deficit, the authors wished to determine whether the large dosage of fentanyl might contribute significantly to the decrease in blood glucose, The authors examined 20 infants who underwent cardiac surgery with small dosage of fentanyl(Group I, n= l0) or large dosage of fentanyl (Group II, n=10), none of whom received dextrose in the clear cardiopulmonary bypass pump prime, maintenance iv fluids, or cardioplegic solution. Blood samples were obtained after induction, during cardiopulmonary by-pass, after cardiopulmonary bypass and after operation. There was no hypoglycemia during the entire surgical periods in all patients. During cardio-pulmonary bypass in group I and group II, after cardiopulmonary bypass, after operation in group II, blood glucose levels were significantly decreased compared with after induction values(p<0.05). Compared with group L the rates of changes from the values after induction were significant during cardiopulmonary bypass and after operation in group II(p<0.05). In conclusion, large dosage of fentanyl is valuable in control the hyperglycemia during open heart surgery in infants.