The metabolic changes in connection with fasting, anaesthesia and surgery in diabetic patients and non-diabetic patients are reviewed. Various perioperative forms of treatment are described. The forms of treatment most commonly employed are infusion of glucose-insulin-potassium (GIK) and subcutaneous administration of insulin followed by infusion of glucose (KON). The more intensive GIK regime provides the diabetic patient with a biochemical regulation which resembles that found in non-diabetics. It has not been proved whether this marginal regulation influences the well-being, morbidity or mortality of the patients. In critically ill patients or patients with concurrent diseases, the GIK regime is recommended as this provides optimal regulation of the diabetes. In the remaining patients, local conditions will influence the choice of form of treatment.