The influence of acute hypoxemia on blood fibrinolytic activity was investigated in 12 healthy males. Physiologically significant hypoxemia was produced by inspiration of 13% oxygen for 30 min. Six healthy males were exposed to hypoxemia at rest and 6 males to hypoxemia during exhaustive physical exercise on an ergocyclometer. During control experiments both groups of health males inspired 21% oxygen. In 5 patients with manifest respiratory insufficiency the effect of hypoxemia at rest was studied during withdrawal of oxygen treatment for up to 2.5 hours. No increase in fibrinolytic activity (measured with euglobulin clot lysis and fibrin plates) due to hypoxemia was observed either in resting healthy males or in patients. In healthy males the increase in fibrinolytic activity after physical exercise at 13% oxygen was even somewhat lower compared to 21% oxygen. No changes in other hemostatic parameters (activated partial thromboplastin time, factor VIII-related antigen, fibrinogen, plasminogen, alpha-2-antiplasmin, fibrin(ogen) degradation products) that could be attributed to hypoxemia, were observed in any group tested. It was concluded that short-term acute hypoxemia does not increase blood fibrinolytic activity in man.