Glucose is a major energy source for all mammalian cells, and it is the principal source of energy for the brain. A constant supply of glucose must be provided in order to ensure against hypoglycemia and the potentially cataн strophic effect this can have on cells of the nervous system. Continual ingestion of a carbohydrate-rich diet can provide the glucose, but prolonged periods of fasting, as in sleep or protracted exercise, can place the organism at risk. Also, restraint on hyperglycemia, which has its own set of deleterious consequences, must be exerted. An intricate mechanism for maintaining the blood glucose within a relatively narrow range has evolved to accomplish these purposes. This involves the production of glucose by the liver, from glycogenolysis and gluconeogenesis, and the peripheral clearance of glucose by tissues such as the skeletal muscle, adipose tissue, and the splanchnic bed,