The experimental studies were carried out to examine the relation between the hemorrhagic tendency after massive transfusion of preserved blood and serotonin. The results were as follows: 1) After massive transfusion of preserved blood, whole blood serotonin and platelet serotonin levels were decreased, while plasma serotonin levels were slightly increased. Tissue serotonin levels were increased in the lungs and slightly elevated in the liver, but there was no change in the small intestines. 2) After massive transfusion of preserved blood, the 5HIAA levels in the urine were decreased. The same tendency was also observed in surgical cases without the transfusion. This change was correlated with post-operative oliguria. 3) The effect of the reduction of bleeding time by administration of synthetic serotonin was not constant, but passing. 4) In coagulation studies on serotonin added to blood in vitro, no effect was found concerning coagulation time, prothrombin time, recalcification time or clot retraction test. On the thrombelastogram, it was observed that coagulation was accelerated in lower serotonin concentration, while suppressed in higher serotonin concentration. 5) In preserved blood, whole blood serotonin and platelet serotonin levels were decreased. On the contrary, plasma and red cell serotonin levels were increased as time passed. 6) From the experimental results above, it is concluded that serotonin contained in platelets should not be recognized as one of the factors causing hemorrhagic tendency after massive transfusion of preserved blood.