Involvement of the kinin system was studied in 58 patients during extracorporeal circulation in open heart surgery. At the onset of extracorporeal circulation, there was no increase of free kinin in blood and no reduction of plasma kininogen, when the latter was expressed as mug bradykinin/mg plasma protein. With the lapse of time, kinin in the blood increased significantly, and decrease of kininogen was also significant. The longer the circulation time, the greater the consumption of kininogen. The site of the kinin formation was presumed to be the heart-lung machine since the level of kinin increased slightly and kininogen decreased gradually in the blood leaving the machine, compared with those levels entering the machine. The difference in kininogen was significantly different from zero at termination of extracorporeal circulation. Administration of a kallikrein-trypsin inhibitor, trasylol (A. G. Bayer), infused into the heart-lung machine, prevented the decrease of kininogen. Reduction of the total peripheral resistance during this circulation was also prevented. The hemoconcentration, presumably the result of vascular permeability increase caused by increased kinin, was prevented by trasylol in cases in which extracorporeal circulation lasted over 60 minutes. Trasylol applied thus appears to be an effective counteragent for circulatory disturbances which occur during extracorporeal circulation.