In the experiment of massive transfusion of compatible preserved blood on dogs, Na-, Cl-, K-, total Ca and Ca-ion-concentration were determined, and electrocardiograms were taken of each case as reference simultaneously. These electrolytes were also determined on patients needed massive transfusion during surgery, and Na- and K-concentration in plasma of preserved blood were traced day by day. Results obtained are as follows: 1) In the blood stored in ice box of 2° to 5℃ of temperature the serum potassium was increased, on the contrary lowered the serum sodium. 2) On massive transfusion of preserved blood, concentration of serum electrolytes of recipients showed minimum fluctuation. 3) The fluctuation of electrolytes was not significantly related to the transfusion rate, nor the preservation period of blood transfused. 4) Serum calcium-ion level was determined on the patients showed hemorrhagic tendency during surgery, but no noticeable correlation was observed. 5) The electrocardiograms showed no significant findings. 6) In the control experiment of fresh blood transfusion, the serum electrolytes did not show marked changes. From the results obtained above, it is concluded that massive transfusion of preserved blood is not dangerous in a sense of serum electrolyte level, although it is preferable to transfuse fresh blood in the following cases; cases in the conditions which will cause hyperpotassemia, infant, cases under hypothermia and impaired calcium metabolism, or cases with circulatory disturbances.