For the purpose of recommending appropriate fraction of inspired oxygen(F1O2) for long term mechanical ventilation support by evaluating the effect of changes in F1O2 on pulmonary shunt ratio in respiratory intensive care, the effects of various F1O2 on pulmonary shunt ratio were investigated and analysed with 20 pediatric intensive care patients who had received open heart surgeries for their congenital heart diseases in the department of Pediatrics, Seoul National University Hospital. The results were as follows ; 1. Through the whole process of control of mechanical ventilation the arterial CO2 tension(PaCO2) showed hyperventilation with the average of 31-34mmHg and pH showed pure respiratory alkalosis with the average of 7.45-7.51. As the fraction of inspired oxygen decreased, the arterial oxygen tension decreased. 2. Inspite of the change of F1O2, analysis of central venous blood gas showed no significant change. 3. The alveolar-arterial oxygen tension difference(AaDO2) was positively related to F1O2. 4. The pulmonary shunt ratio(Q's/Q'T) showed no significant difference in the F1O2 range of 0.6 to 1.0. But the Q's/Q'T in the F1O2 range of 0.3 to 0.4 decreased significantly comparing to that of 0.6 to 1.0 F1O2 range. Linear regression analysis of Q's/Q'T showed that the Q's/Q'T was positively related to F1O2 and AaDO2, respectively.