State of the anticoagulation system was studied after intravenous administration of trypsin at doses similar to the concentration of alpha-thrombin, activating chemoreceptors of vascular walls. Trypsin at doses 0.5 microM-3.7 microM did not affect the anticoagulation system as indicated by unaltered rate of nonenzymatic fibrinolysis. Occurrence of trypsin in blood led to generation of thrombin, which caused limited proteolysis of fibrinogen with subsequent increase in content of soluble fibrin, but did not stimulate the anticoagulation system. Distinct stimulation of the enzymatic fibrinolysis resulted from both liberation of plasmin due to direct proteolysis of plasminogen and unspecific release of the plasminogen activator after destruction of vascular endothelium. High doses of alpha-thrombin (70 NIH un per 1 ml of the preparation) did not activate the anticoagulation system but the total fibrinolytic activity was increased die to elevation in the ratio of enzymatic fibrinolytic activity. The data obtained suggest that the proteolytic activity of thrombin and trypsin is not responsible for the reflectory reaction of the anticoagulation system. High specificity of alpha-thrombin, caused by the presence in its structure of the recognizing sites of high molecular substrates, enables the enzyme to interact with chemoreceptors of vascular walls and to stimulate the anticoagulation system.