The effects of infusion of the oxygen radical generating system, hypoxanthine-xanthine oxidase, on pulmonary hemodynamics and plasma proteolytic systems, as the plasma kallikrein-kinin and fibrinolytic systems, were studied in pigs by infusion of xanthine oxidase into the pulmonary circulation. A marked pulmonary vasoconstriction and increase in vasculatur resistance with a maximum after 25 min was observed. The effects were attenuated by indomethacin, allopurinol and catalase. During the 130-min observation time there were no signs indicating activation of either the plasma kallikrein or the plasma fibrinolytic systems.