If one injects bovine thrombin intraaortically into dogs, a number of changes take 1-3 place. There is a precipitous fall in the aortic blood pressure which gradually returns to relatively normal levels over a 15to 30-minute period. This is, under proper circumstances, only temporary, and a secondary fall soon begins which terminates fatally within 12 to 24 hours. Autopsy regularly shows a hemorrhagic necrosis of the gastrointestinal mucosa which is typical of canine shock. Blood coagulation changes are interpreted as indicative of an episode of intravascular coagulation. There is a rapid and dramatic fall in level of blood fibrinogen which is marked within 20 minutes but returns to normal or supranormal levels within four to six hours. There is a marked but temporary fall in platelets. Prothrombin time is prolonged. These changes are probably caused by the using up of the blood coagulation factors in an intravascular clotting episode. In