In order to clarify the origin and release mechanism of plasminogen activator in the tracheobronchial secretion of rats, electrophoretic analysis of the secretion and studies on the effects of certain vasoactive drugs on the activator activity in the secretion were carried out. From the results of electrophoretic analysis of the tracheobronchial secretion in non-treated rats, protease inhibitor composed of glycoprotein was not contained in the secretion in contrast to the circulating blood, but a protein of low molecular weight like albumin in the circulating blood was contained in the secretion. Furthermore, after injection of noradrenalin, the blood pressure was temporarily elevated and the fibrinolytic activity of the euglobulin fraction in the circulatory blood was also increased. Subsequent to this elevation of fibrinolytic activity in the circulating blood, the fibrinolytic activity in the tracheobronchial secretion increased. Based on these results, it is suggested that the increase of fibrinolytic activity in the circulating blood was due to increased release of plasminogen activator from the vascular wall, and the increased fibrinolytic activity of the tracheobronchial secretion was caused by a consequent increased transudation of plasminogen activator from the circulating blood into the tracheobronchial lumen.