Hourly plasma glucose concentrations in 191 diabetic patients undergoing 200 operations were measured. The glucose infusion rate was controlled. Insulin-taking diabetics given no insulin or a fraction of their usual dose preoperatively developed rising plasma glucose concentrations beginning with the start of operation. The mean rate was 22 mg/dl per h (no insulin) and 17 mg/dl (1/2-1/4 the usual dose of insulin). Of the patients, 8% achieved plasma glucose concentrations > 400 mg/dl. Patients given regular insulin during the operation had no hourly rise in plasma glucose. Hypoglycemia occurred in 5.5% of these patients. Arbitrary management regimens should be abandoned. Plasma glucose levels should be measured frequently and insulin and/or sugar should be given to each patient as needed.