Through the use of a feedback-controlled dextrose infusion system, we obtained continuous monitoring of the blood glucose level in a patient undergoing surgery for multiple pancreatic beta-cell tumors. In addition, this device infused dextrose at variable rates to maintain a predetermined euglycemic level of 90 mg/dl. Before locating the source of excessive insulin production, the maximum dextrose infusion rate of 400 mg/min was required; but after extirpation of multiple insulinomas, the dextrose-infusion rate declined whereas the blood glucose level rose above the preselected level. These results emphasize the usefulness of a monitoring and infusion system not only in protecting the patient from the hazard of hypoglycemia under anesthesia but also as an aid in determining whether all insulin-secreting tumors have been removed.