It is obvious that in the field of surgery the management before and after operation has a grest influence on the survival of the patient. As a step in such management the acid-base balance is considered to have a great bearing on the homeostasis of the body. In view of this, the author measured the humoral pH of the patients for the period of one week before and after operation with Astrup's microequipment to determine various values of the acid-bese balance. Simultaneously, measurements were taken of the circulating blood volume and Ht values of the same individuals, and in some cases the contents of Na, K, CI, lactate and pyruvate in their serum and urine were estimated. As a result it was found that even in the case of acid-base imbalance postoperative metabolic acidosis has an important role in it. Especially low pH was observed in the cases of advanced age of 60 years, requiring more than 6 hours for operation of malignancy, and needing blood transfusion of over 1,000 cc, that is, the greater was the surgical intervention, the more severe was postoperative metabolic acidosis. The circulating blood volume decreased immediately after operation, and taking a course similar to the change in the acid-base balance, it returned to the preoperative level in one week. Ht value also served as a good criterion for the circulating blood volume. while the blood Na level declined after operation, the potassium and chloride levels showed no appreciable change. Lactate and pyruvate increased immmediately after operation, but they returned to the preoperative level within 3 days. Judging from these results, Lactate-Ringer solution was beter than 5% glucose solution for the transfusion during operation, especial effective was Lactate-Ringer solusion at pH 8.0 for the prevention of postoperative metabolic acidosis. In addition, from standpoint of mainten anee of the circulating blood volume, D40 Lactate-Ringer solution (Lactate-Ringer solution supplemented with low molecular Dextran) proved to be effective. For the treatment of this metabolic acidosis THAM (Trishydroxy-methylaminomethane) was effective as reported by various authors. The metabolic acidosis in 2 or 3 days was not so severe as to require any correction.