It has been shown that changes of serum electrolyte have occurred during cardiopumonary bypass and that the changes have affected on prognosis of open heart surgery. This study was undertaken to reveal causes of the electrolyte changes and to minimize them. Clinical study was performed in 48 patients who underwent open heart surgery for either congenital or aquired heart disease. Experimental study was made using 30 healthy mongrel dogs and a main purpose of the experiment was to determine mutual changes of the electrolyte, including sodium, potassium and magnesium. when magnesium was added in the priming solution Heart-lung machine employed was disc oxygenator type (Pemco) or bubble oxygenator type (Junken), primed with hemodiluted solution (20%) under normothermia. The following results were obtained: -Clinical studies- 1) Serum potassium and magnesium levels decreased during perfusion, but potassium level of R. B. C. increased a little. Urinary excretion of potassium increased after perfusion. However, it started during perfusion in a group (group B) in whom digitalis, KCl and diuretic had been given before operation. Potassium of the serum and R. B. C. returned to the preperfusion level by the 1st post-op. day and magnesium level did by the 4th post-op. day, but in group B it returned after the 4th post-op. iday. 2) Hypokalemia during perfusion became severer as perfusion time was prolonged. 3) Open heart surgery resulted in metabolic acidosis but serum potassium level was not in correlation with serum PH. 4) Potassium and magnesium levels in the muscle decreased after perfusion, while sodium level increased. -Experimental studies- 1) Potassium and magnesium levels of the serum and R. B. C. decreased as well as in the clinical studies. 2) In the cerebrospinal fluid, however, potassium and magnesium levels were gradually risen during perfusion. 3) Potassium level of the striated muscle decreased during perfusion, but there was no change in potassium level. Potassium level of the liver remained unchanged. 4) Magnesium supplements during perfusion maintained electrolyte balance satisfactory, namely, urinary loss of potassium reduced and, therefore, hypokalemia was improved. It was concluded that magnesium supplements were recommended to be added to priming solution to improve potassium metabolism during and after extracorporeal circulation.