About on hemorrhagic tendency following massive transfusions of preserved blood, dynamics of mesenteric capillaries, changes in capillary resistance and platelets, and bleeding time were studied experimentally and clinically. Following transfusions, peripheral capillaries were dilated and the blood stream become slow and pooling. Endothelial damages, increase in the permeability and escaping of red blood cell through the wall were observed. Capillary resistance was decreased in the early stage of transfusions and was closely related to the injected blood volume. Striking decrease in number of platelets and prolongation of bleeding time were also recognized in almost all of the cases. It was surmizd that the main factor of hemorrhagic tendency was increase in the permeability or the fragility predisposed by changes in capillary wall with reduction in platelet counts due to blood transfusion itself.