Bleeding tendency following extracorporeal Bleeding tendency following extracorporeal circulation was studied mainly in the view of vascular disturbances. 1) Pre-and post-operative changes in hemostatic mechanism were investigated in 61 patients, who underwent open heart surgery under extracorporeal circulation with Kay-Anderson heart lung machine. After perfusion the most noticeable and constant changes in hemostatic mechanism were prolongation of bleeding time, decreased capillary resistance, increased capillary permeability and reduction of platelet count which showed vascular disturbance. No correlation was seen between these changes and perfusion time. Among our clinical series there were 2 cases with spontaneous petechiae which, with certainty, were due to bleeding tendency caused by vascular disturbances. 2) Platelet count was markedly reduced during perfusion and return rapidly to normal after neutralization of heparin. This decrease of platelet count was proved to be partly caused not only by mechanical destruction of heart lung machine, but also by heparin itself. Beside vascular disturbances due to peripheral circulatory insufficiency caused by perfusion (atony of peripheral vessels, stagnation of the blood and erythrocyte sludging etc.), heparin was revealed to be a factor of postperfusional vascular damage. For prophylaxis of vascular disturbances due to peripheral circulatory insufficiency, low molecular weight dextran was found effective. Prolongation of bleeding time was considered to be developed because of decreassed capillary resistance. 3) Heparin was proved to have side-effects of decreasing platelet count and capillary resistance, and of increasing capillary permeability. The decrease of platelet count was experimentally found to be reversible and the mechanism of this deficiency was attributed to platelet clump formation by heparin. Decrease of capillary resistance was elicited to be due to dysfunction of the vascular endothelium caused by heparin. Because it was found that heparin had effects to develop petechial hemorrhage, leucocytic sticking to the endothelium and increased extravasation of leucocytes. Prolonged bleeding time and decreased capillary resistance were observed in rabbits in which only platelet count deliberately decreased. Therefore, there must be correlation among them, but it is conceivable that capillary resistance decreased by heparin might be not only due to decrease of platelet count, but also to direct influence on capillary vessels by heparin.