Serial blood glucose and plasma immunoreactive insulin measurements during operations for insulomas were clinically useful in six patients. Mean blood glucose concentration rose by 40 mg/100 ml/hr during the initial 30 minutes after tumor removal, when no further functioning insuloma remained in situ. This information was available to the surgeon with a 15-minute delay. Insulin levels determined subsequently confirmed the assumptions based on glucose measurements. In addition to helping the surgeon to resect the least amount of pancreas necessary, these measurements enhanced the patient's safety from potentially dangerous hypoglycemia during anesthesia. Preoperative celiac arteriograms were done in five patients. They were helpful in localizing tumors in two patients and were retrospectively recognized to have correctly indicated the location of the tumor in a third patient.