One of common factors of bleeding tendency during and after extracorporeal circulation is due to fibrinolysis. Moreover, it is a well known fact that kinin-system, which is activated by fifibrinolysis, Hageman factor, trypsin and tissue activator, is responsible for pathological changes of hemodynamics in shock, inflammation, pancreatitis etc. Therefore, it would readily be conceivable that kinin-system was also activated by extracorporeal circulation through the activation of fibrinolysis and Hageman factor, since extracorporeal circulation activates fibrinolysis and Hageman factor. This study is to report activations of kinin-system in 37 patients who underwent open heart surgery by determing plasma kininogen (kinin precursor). The results obtained were as follows: 1. Activation of fibrinolysis during and shortly after extracorporeal circulation was observed. 2. Plasma kininogen was markedly reduced during extracorporeal circulation and resumed to normal after extracorporeal circulation, which would be suggestive of release of kinin. 3. AMCHA (50mg/kg) inhibited fibrinolysis due to extracorporeal circulation and fibrinolysis returned to normal 180 min. post-operatively. 4. Trasylol of 10,000 units per kilogram was equivalent to AMCHA of 100,000 units for a child and 200,000 units for an abult were often suboptimal. 5. Trasylol was also effective in suppressing occurrence of kinin activation.