Ketone body (3-hydroxy butyric acid) present in the blood was successively examined during surgical operations to ascertain the condition of energy metabolism and its clinical significance. In 10 patients (R group) to whom only the lactated Ringer's solution was infused during the operation, values of 3-hydroxy butyric acid increased remarkably in the 30 minutes after the operation began. In 7 patients (GI group), to whom the lactated Ringer's solution in 5% of glucose was administered, the values also increased in the 30 minutes after the operation was started. However, after the administration of glucose the values decreased, and returned to the normal value with administration of insulin. Furthermore, blood sugar readings which were examined at the same time increased significantly immediately after the beginning of the operation in both groups. Insulin readings showed considerably low values during the operation in R group. The readings of free fatty acid tended to show high values in R group, while they tended to decrease after the dose of glucose in GI group. With regard to epinephrine and norepinephrine readings, both groups showed significantly high values 30 minutes after the start of operation.