This experiment was carried out to study the change of potassium before, during, and after extracorporeal circulation. Serum potassium and urinary potassium excretion were examined in 7 cases of patient who were taken open heart surgery using extracorporeal circulation. All of 7 cases were administered 10-25mEq of KC1 during extracorporeal circulation. Serum potassium was not changed significantly during, and after operation, but urinary potassium excretion was declined during extracorporeal circulation and tend to rise after extracorporeal circulation. This result showed no remarkable relation of serum potassium and urinary potassium excretion.[KTCS 1978;3:296-302]