Experiments were made on 20 cats to show that within the first minutes intravenous administration of thrombin led to the enhancement of blood coagulation, an abrupt lowering in fibrinogen concentration, growth of antithrombin III activity and inhibition of fibrinolysis. 30-60 minutes after thrombin injection there developed a secondary hypocoagulation accompanied by the increment of the level of natural anticoagulants and by drastic enhancement of fibrinolysis. In animals given timalin in a dose of 2 mg for 5 days, the injection of thrombin produced a more remarkable increase in blood fibrinolytic activity.