In the experiment of massive transfusion of preserved blood on dogs, Na- and K-concentration in blood cells, Na-, Cl-, K- and Ca- concentration in urine, and water volume and Na-, Cl-, K- and Ca-contentration in tissues were determined. Results obtained are as follows: 1) Following blood transfusion, Na-concentration was decreased and K-concentration increased in blood cells of recipients. 2) After blood transfusion, Na- and Cl-concentration were decreased, K- and Ca-concentration increased in the urine. 3) Water content was increased in every tissue investigated after blood transfusion. 4) After blood transfusion, Na-, Cl- and K-concentration in tissues were increased, Ca-concentration decreased. 5) In the control experiment of fresh blood transfusion, the changes of Na- and K-concentration in blood cells, in urine and in tissues were nearly neglectable. From the above, it was concluded that the serum electrolytes were well regulated in mechanism of their intercurrence and excretion intra and extra-cellularly.