In dogs, immediately after acute bleeding of 50, 30 and 10 cubic centimeters per kilogram of body weight, equivalent amounts of either blood or dextran solution were infused intravenously. The changes in serum proteins as well as the effects of pharmacological hibernation and of corticosterone following such treatments were observed. 1) Decrease of total protein, particularly of albumin, and increase of α2-globulin were noted after blood transfusion exceeded 30cc/kg. Other protein fraction did not show any changes. It required more than 4 weeks in the 50cc/kg blood transfusion group for complete recovery, while in the 20cc/kg group the required recovery time was 3 weeks. These changes were more remarkable under condition of pharmacological hibernation. Corticosterone inhibited the decrease of albumin and total protein; upon discontinuance of this drug the decrease again occurred, followed by a slow recovery. 2) When dextran solution was infused under similar conditions, the changes in serum protein fractions were also observed, but the recovery time was distinctly shortened. Further pharmacological hibernation failed to show any effect.