The dependence of cerebral energy metabolism upon glucose, 3-hydroxybutyrate, and lactate as fuel sources during the postnatal period was investigated. The brain of 6 day old suckling pups used very little glucose, but by the 15th postnatal day glucose was the major catabolite. Hydroxybutyrate was not a major brain fuel at either 6 or 15 days of age. Its utilization accounted for only 19% of the brain's total energy needs at 15 days of age, even through blood ketone concentrations are near maximal at this time. Seventy percent of the cerebral metabolic requirements were met by lactate in animals aged 6 days. The major role played by lactate as a substrate for brain metabolism in young pups was not a result of abnormally elevated blood lactate concentrations. The slow catabolism of glucose in young brain can not be explained by low rates of influx or inadequate enzymatic capacity.