The level of serum potassium concentration is very important aspect in postoperative cardiac patients The postoperative cardiac arrhythmia and digitalis intoxication are known to be closely related with hypokalemia and also to cause the irreversible cardiac dysfunction. In this study, the changes of the level in serum and urine concentrations during, after extracorporeal circulation[EGG], Predict and Postdict periods are analyzed and compared statistically with postoperative cardiac patients 46 persons according to divided 8 groups. 1. There was no difference significantly in concentrations of serum and urine potassium in each period according to age, sex and disease types. 2. There was no difference significantly in concentrations of serum and urine potassium in each period according to the use of normothermia and hypothermia. 3. There was no difference significantly in concentrations of serum and urine potassium in each period according to the length of extracorporeal circulation time and aortic cross clamping time. 4. There was no difference significantly in concentrations of serum and urine potassium in each period according to the amount of the infused cardioplegic solution and level of Hct.