The energy-rich substrates available to the fasting stressed baboon neonate and infant are quantitatively similar to the metabolic fuels presented to the stressed low birth weight human newborn. Within a few hours after birth, fasting arterial plasma glucose levels in the baboon neonate approximate those of 4-6-week-old baboon infants after a 20-hr fast. Lactate levels are high and comparable for both age groups. In contrast, beta-hydroxybutyrate is quite low in the immediate neonatal period, but rises to significantly higher levels (P less than 0.001) after a fast at 4-6 weeks. In addition, glycerol levels are higher (P less than 0.02) in the fasted older infant compared with the fasting neonate. Computation of mean cerebral blood arteriovenous differences and oxygen equivalents for animals studied in the first 50 hr of life demonstrates that glucose uptake can account for 50% or less of cerebral oxygen consumption in the newborn period. In confirmation, the respiratory quotient in these animals is 0.52 +/- 0.06. Cerebral oxygen consumption in the immediate neonate is greater than can be explained by utilization of glucose and the small quantities of acetoacetate and beta-hydroxybutyrate available at this time. At birth, cerebral uptake of lactate is noted, but this phenomenon is not observed at 6 and 12 weeks of age.