To evaluate the usefulness of maltose as an energy substrate to be administered during surgery, five per cent maltose in lactated Ringer's solution and five per cent glucose in lactated Ringer's solution were administered to 10 cases each, at a rate of 5 ml . kg-1 hour (0.25 g . kg-1 hour as sugar) for two hours from the start of oral surgery, and their metabolic effects were compared. The maltose group showed a smaller increase in blood sugar level than the glucose group immediately after the completion of infusion. The mean plasma concentration of maltose reached a maximum of 121.6 mg/dl, and it remained at 12.3 mg/dl at four hours, indicating that the retention time of maltose in blood was longer than that of glucose. The mean recovery of sugar from four-hour urine samples was 3.26 per cent in the maltose group and 0.06 per cent in the glucose group respectively, showing greater urinary excretion by the maltose group. Plasma insulin was elevated less after maltose than after glucose infusion. The elevation following maltose infusion was considered not to be due to the administration of maltose per se, but to glucose produced from the maltose in the body. The anti-ketogenic effect of maltose was comparable to and tended to last longer than that of glucose. From overall assessment it was concluded that maltose exerts essentially the same metabolic effects as glucose when used under these conditions.