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-1.h-1 (no insulin) and 17 mg.dl-1.h-1 (one-half to one-fourth the usual dose of insulin). Eight per cent of the patients achieved plasma glucose concentrations greater than 400 mg/dl. Patients given regular insulin during the operation had no hourly rise in plasma glucose. However, hypoglycemia occurred in 5.5 per cent of these patients. The authors suggest that 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.