SummaryGlucose oxidation by developing rat intestine changed dramatically during the period of suckling and weaning. After weaning, glucose oxidation to CO2by intestinal slices increased over 3-fold. This was associated with an increase in lactate production from glucose and an increase in the rate of pyruvate decarboxylation.Active pyruvate dehydrogenase in intestine of developing rats also increases in activity at the time of weaning, suggesting that the suppression of glucose oxidation during the suckling period is controlled by pyruvate dehydrogenase. Glucose oxidation to CO2and pyruvate decarboxylation to CO2by intestinal slices of postweaned animals was inhibited by exogenous 3-hydroxybutyrate. But exogenous 3-hydroxybutyrate did not inhibit glucose and pyruvate oxidation in intestine of suckling animals which have higher levels of endogenous 3-hydroxybutyrate than intestine of postweaned rats. Palmitate, in contrast, inhibited glucose and pyruvate oxidation by both pre- and postweaned intestine.