We found that the activity of biotinidase is much lower in human and rat brain or human CSF than in serum or other tissues that have biotin-dependent carboxylase activity. The brain seems to be unable to recycle biotin and depends on biotin transferred across the blood-brain barrier. The biotin-deficient state that results from an inherited lack of biotinidase results in a moderate decrease in brain pyruvate carboxylase activity. This is followed by more severe accumulation of lactate in brain than in other organs, which may explain why affected children have neurologic symptoms before many peripheral features.