To 48 adult patients general and spinal anethesia was administered for elective orthopedic (intra-thoracic and extremity) and abdominal(general and gynecological) operations. Radial artery was cannulated and postoperative change of PaO2 was observed for 7days. The results are as follows: 1) General anesthesia administered for operations on the legs results in a reduction of PaO2 which is maximal immediately after discontinuation of anesthesia, gradually returns toward normal in a 3-hour period, and becomes normal on the first postoperative day. 2) General anesthesia administered for thoracomy and laparotomy is followed by the same early changes, which do not return toward normal in the first 3 hours. Reduciton of PaO2 persists, and PaO2 continue to deteriorate for several days, not completely returning to normal even 7days post-operatively. 3) When spinal anesthesia is administered for laparotomy, PaO2 does not begin to fall untill several hours after the end of operation. The subsequent course follows as that in 2, above. 4) When spinal anesthesia is administered for operation on the legs, PaO2 does not change significantly throughout the postoperative period. 5) Thus, it is concluded that late forms of postoperative hypoxemia is influenced primarily not by the method of anesthesia, but by site of operation.