A Clinical study was carried out to observe changes in concentration of serum and urinary potassium during and after open-heart surgery under extracorporeal circulation with Heart-Lung-Machine. The patients studied in the present work included eight cases of congenital heart anomalies and seven cases of acquired valvular diseases of the heart. Blood and urine samples were collected at intervals of 30 minutes during open-heart surgery and at intervals of 8 hours until 48 hours after the operation to determine concentration of serum and urinary potassium. 1.While the concentration of urinary potassium was not changed during the operation, the concentration of urinary potassium was rapidly increased after the operation. Eight hours after the operation, the concentration of urinary potassium was increased to the highest value of 86.97±43.96 mEq/L and was gradually decreased thereafter. The concentration of urinary potassium, however, still remained elevated even 48 hours after the operation, the concentration being greater than preoperative state by 19.90 mEq/L. 2.The concentration of urinary potassium after the operation was higher in the group who went through 90 minutes of extracorporeal circulation [108.71±94.71 mEq/L] than in the group who went through 60 minutes of extracorporeal circulation [86.58±50.87 mEq/L]. The Increased level of urinary potas-sium observed In the group who went through 90 minutes of extracorporeal circulation continued for longer period of time as compared with that in the group who went through 60 minutes of extracorporeal circulation. 3.The increased level of urinary potassium was observed in the both groups of patients with or without diuretics plus digitalis therapy. However, the increased level of urinary potassium appeared to con-tinue for longer period of time in the group of patients receiving diuretics plus digitalis therapy prior to the operation than in the group of patients receiving no diuretics plus digitalis therapy.[KTCS 1982;1:40-45]