Fibrinolytic system was studied in rats under various conditions of intravenous administration of the plasminogen tissue activator (RTA). Two administrations of the activator with an interval of 10 min did not cause an additional activation of fibrinolysis as compared with single treatment. Increase in amount of administrations up to four times within a day (an interval for 30 min) as well as chronic administrations of PTA within 5 days did not cause also any significant and long-term action on the animal fibrinolytic system. Efficiency of blood plasma antiactivator was decreased after repeated and chronic PTA administrations. Insensitivity of the healthy animal fibrinolytic system to repeated and chronic administration of PTA depended apparently on binding of the activator to inhibitors and on its rapid excretion from circulation.