The influences of splenectomy upon blood coagulation in hepatic vein and portal system were studied. 1) After splenectomy the blood coagulating time is generally prolonged and it is especially marked in hepatic vein and portal system. It is caused mainly by decrease of fibrinogen and partly by thrombin. 2) The splenectomy does not show its marked influence to prothrombin time, calcium quantity and number of blood platelet. To konw why the blood coagulating time is shortest in V. gastroepiploica among portal system excepting that in V. portae, the influences by resection of omentum majus upon dlood coagulativity in hepatic vein and portal system were studied. 1) The blood coagulating time in portal vein is markedly prolonged by resection of omentum majus. This is considered to have owed to the decrease of fibrinogen produced in reticuloendothelial system. 2) In normal dogs the blood coagulating time in portal vein is markedly longer than that in V. lienalis, V. mesenterica superior and V. mesenterica inferior. The reason is considered to be the addition of blood from stomach which contains blood from omentum majus.