It has been shown that blood glucose level was risen during cardiopulmonary bypass procedure. However it is uncertain whether this alteration in carbohydrate metabolism in the result of inadequate insulin release or of an abnormality in glucose metabolism. This report concerns glucose metabolism and insulin release as well change in serum potassium concentration in patients undergoing cardiac surgery under moderate hypothermic cardiopulmonary bypass. Hyperglycemia and delayed insulin response to glucose may be due to an increase in catecholamine retion induced by the surgical trauma and hypothermic cardiopulmonary bypass. Plasma glucose, insulin, serum sodium and potassium concentrations were measured in 14 patients undergoing open heart surgery with moderate hypothermia. Patients were anesthetized with fentanyl 10~20 pg/kg, droperidol and nitrous oxide in oxygen. Priming solution of bypass was composed with heparinized whole blood, Hartmann's solution and 5% dextrose water added at the ratio of 2:1, as a result of hematocrit 28~30% during bypass A significant in blood glucose concentration occurred with bypass from the control value of 87.0±7.85 mg/dl to 529.8±67.52 mg/dl in children and from 88.8±10.42 mg/dl to 474.8±62.27 mg/dl in adults (p<0.05), partly as a result of the stress hyperglycemia and exogenous glucose load from the priming solution. Following bypass the blood glucose remained above the preanesthetic concentration at the level of 237.2±57.57 mg/dl in children and 210.4±45.29 mg/dl in adults, and this elevation persisted into the period following surgery (p<0.05). Plasma insulin concentration remained low in spite of increased glucose concentration at the level of 7.8±2.79 μl.U/ml in children and 10.1±3.84 pI.U/ml in adults and start to elevate with the start of bypass. The highest concentration at 39.6±13.90 μl.U/ml in children and 40.5±16.97 μl.U/ml in adults occurred following hypothermia and this elevation persisted during bypase procedure(p<0.05). Serum sodium and potassium concentration decreased significantly at the level of 134.0±3.58 mEq/ L and 3.1±0.40 mEq/L in children and 128.9±4.49 mEq/L and 3.2±0.37 mEq/L in adults, respeetively, following bypaas procedure (p<0.05). These observations indicated that exogenous glucose load such as glucose contained priming solution and cardioplegic solution may aggravate the stress hyperglycemia.