Using a Biostator glucose-controlled insulin infusion system to monitor blood glucose during surgery, we have shown that both nondiabetic and diabetic patients have a tendency towards hyperglycemia during surgery. This appears to be due to suppression of endogenous insulin secretion as measured by serum C-peptide levels. Some diabetic patients maintained relatively normal glucose values during surgery when infused with saline alone and not given glucose or insulin, but 2 of 5 were not well controlled by this means. Hyperglycemia in both diabetic and non-diabetic patients was related to the rate of infusion of exogenous glucose. The biostator glucose-controlled insulin infusion system could be used in feedback mode as an apparently safe and effective means of controlling blood glucose during surgery on diabetic patients.