The group of pulmonary resections under normal blood pressure and the group of those under endotracheal anesthesia of closed circulation type were compared. The arterial blood gas content was measured at the pre-operative stage, directly after the intubation, during operation and after recovery using Van Slyke manometer. The results were as follows: 1) The O(2) content of the arterial blood showed a decreasing either under normal blood pressure of under hypotensive group, most strikingly during the operation moderately after recovery, and slightly directly after the intubation. The mean values of both groups were compared. The O(2) content of the arterial blood had more definite tendency of decreasing in the hypotensive group than the normal group, but it was not significant. 2) The CO(2) content in the arterial blood in both groups generally decreased slightly directly after intubation and most strikingly during the operation. After recovery it became rather more in the normal group than before the operation, while that in the hypotensive group decreased slightly and still less than before the operation. 3) Throughout all cases we found the decreasing of total blood gases especially during the operation, namely there was a decreasing tendency of general metabolic function of gases, which was regarded to be stronger in the hypotensive group than in the normal group. 4) Though the decreasing tendency of metabolic function of gases was rather stronger in the hypotensive group than that in the normal group, the value of arterial blood was rather better in the hypotensive group. This value in both groups increased with the lapse of the stages of operation. 5) In the cases, in which the arterial blood value more than 3.0 already before the operation, we must be careful to determine the indication of pulmonary resection. We must pay close attention to it, because the patient is apt to fall into risk postoperatively even if we can safely finish the operation.