1. A considerable increase in the plasminogen activator content in the internal jugular venous blood as compared with that in the arterial blood was found in fifty patients. 2. Inhalation of 5% carbon dioxide in air was associated with increased plasminogen activator content in blood from the internal jugular vein in all the eight patients treated in this way. 3. It is suggested that the human brain makes a contribution of plasminogen activator to the circulation of the body and that the amount contributed is determined by the rate of the blood flow.