The site of operation is one of the most significant factors affecting the degree of decreased respiratory function and arterial hypoxemia in the postoperative period. To elucidate the relationship between the site of operation and the postoperative hypoxemia, the authors have performed pre-and post-operative arterial blood gas analysises on the 1st POD through the 4th POD on 30 subjects. The experiment was performed on 3 groups, each of which consisted of 10 subjects; an upper abdominal surgery group, lower abdominal surgery group and extremity surgery group. We have selected only those who have had no preoperative cardiopulmonary problems and excluded those who have developed such complications. The results were as follows; 1) In the upper abdominal surgery group, all postoperative PaO₂, values were found to be decreased significantly. 2) In the lower abdominal surgery group, only the 2nd POD PaO₂ value was found to be significantly decreased. 3) No demonstrable postoperative change of PaO₂, was noted in the extremity surgery group.