The alterations in serum and urine potassium were studied in twenty two patients who underwent open heart surgery using extracorporeal circulation from June 1990 to August 1990 at the Department of Thoracic and Cardiovascular Surgery, School of Medicine, Kei-myung University. There were fifteen cases congenital heart disease and seven acquired heart disease. The serum and urine potassium levels were measured pre-, intra- and postoperatively until seventh postoperative day using ionic selective electrode measuring method. After general anesthesia, the serum potassium level decreased significantly but slowly increased during.cardiopulmonary bypass and returned preoperative level after operation. The urine potassium level decreased slowly from general anesthesia to cardiopulmonary bypass weaning but returned preoperative level following operation. During cardiopulmonary bypass, serum and urine potassium levels in diuretic group were lower than that of non diuretic group. There was no remarkable difference in the serum potassium level between single RA cannulation group and bicaval cannulation group preoperatively, but the serum potassium level in single RA cannulation group was much higher than that of bicaval cannulation group. There was no significant difference in the urine potassium level between single RA can-nulation group and bicaval cannulation group postoperatively.