The aerobic uptake and metabolism of D-fructose in the rings of the small intestine of conventional and germ-free piglets during development was studied. No accumulation of fructose in the intracellular water against its concentration gradient was found. In both groups of animals the intracellular concentration of fructose decreases with increasing age, being highest in the newborn intestine and the sharptest decrease being observed on the first two days of post-natal life. Lactic acid was found to be the main metabolic product of fructose; a considerable amount of it was formed also from endogenous sources of the small intestine. In the newborn piglet fructose does not affect the formation of lactic acid, on the first two days after birth this effect of fructose is already apparent and more pronounced in germ-free piglets than in conventional ones. Conventional and germ-free animals differ markedly in endogenous lactic acid production. While in the former with increasing age endogenous lactate formation decreases, in the latter it rises after a sharp drop on the first post-natal day.