In order to study the changes of blood sugar and electrolyte during cardiopulmonary bypass(CPB) in open heart surgery, 28 patients were selected and observed. The results are as follows: 1) The levels of blood sugar were increased significantly(p<0.01) in all patients compared to the controls as the operation advanced to the end of the CPB. But there was no significant correlation the with the CPB. 2) The increase in blood sugar level was greater in the TOF group and it was at significantly increased(p<0.01) after the CPB and was maintained higher at the end of the CPB. 3) The changes of electrolyte and arterial blood gas values during CPB were not remarkable.