In twenty patients who underwent a knee arthrotomy in which a pneumatic tourniquet was employed, there was a significant increase in fibrinolytic activity (measured by the amount of time for the lysis of euglobulin and by the fibrin-plate method) in the systemic circulation which peaked at fifteen minutes and lasted for thirty minutes after release of the tourniquet. No rebound occurred thereafter. The concentration of fibrinogen and the platelet count decreased and the concentration of the products of fibrin degradation increased after deflation of the tourniquet. Arterial PO2, PCO2, and pH were changed significantly. In contrast, fibrinolytic activity did not increase in patients undergoing operations on the lower extremity without a tourniquet. Deep-vein thrombosis developed in two patients who were treated with a tourniquet and in seven patients in whom a tourniquet was not used. We concluded that increased fibrinolytic activity, presumably mediated through enhanced release of plasminogen activator, might be partly responsible for the decreased incidence of venous thrombosis in the patients in this study for whom a tourniquet was used.