After perfusion of rabbit carotid sinus with prethrombin 1, hypocoagulation was found in the system circulation due to reflectory reaction of the anticoagulation system. Heparin was secreted already within the first minutes of the perfusion with prethrombin 1 (1 . 10(-6) M); maximal activity of the process was observed within 5 min. Liberation of heparin from the vessel wall of carotic artery was not caused by a mechanism of local secretion, as evidenced by absence of the phenomenon in perfusion of the carotid sinus with prothrombin at molar concentration, corresponding to the concentration of prethrombin 1. Perfusion of denervated carotid sinus by means of prethrombin 1 did not stimulate the heparin secretion. Thus, the heparin secretion was regulated by a nervous mechanism, exhibiting the reflectory nature. The data obtained suggest that perfusion of the rabbit carotid sinus with prethrombin 1 stimulates the anticoagulation system and secretion of heparin.