It is well known that the pulmonary capillary endothelium is rich in angiotensin converting enzyme(ACE), which is released in the event of smoking, acute lung injury, or some lung diseases such as Gauchers disease and hypertension. Serum ACE levels may be clinically useful because they are reflections of pulmonary circulation. In order to evaluate the effect of extracorporeal circulation and pulmonary perfusion on serum ACE levels, we measured serum ACE level during prebypass, total bypass(5', 30', 60') and pulmonary perfusion (30', 60', 24 hrs.) in 10 open-heart patients. The results were as follows: 1) The SACE level before the begining of extracorporeal circulation was 10.03±1.66u/ml and decreased significantly to 2.79±0.63u/ml(p<0.005) 5min. after extracorporeal circulation was initiated. 2) The decreased SACE level seen during extracorporeal circulation returned to a nearly normal (9.33±1.8u/ml) 24hrs. after pulmonary perfusion. 3) There were no significant correlations between the SACE level and the variation of age during extracorporeal circulation and pulmonary perfusion. The above results suggest that SACE levels are proportional to the amount of pulmonary blood flow.