We have studied maturation of brain barrier systems in fetal, newborn, juvenile and adult rabbits. We have compared choroid plexus and brain Na+,K+-ATPase levels in each age group, as well as serum and CSF Na+ concentration as a measure of the ability of the choroid plexus to generate a gradient from blood to CSF. The choroid plexus appears functionally mature at all ages studied, both in ability to produce a Na+ gradient and in ATPase levels. In contrast, brain ATPase levels rose markedly with age. Kidney ATPase measured for comparison showed a pattern similar to brain.