An understanding of the development of homeostatic transport mechanisms will enhance our appreciation of the function of this secretory tissue in mature organisms. By investigating drug-induced changes in the distribution of various inorganic ions across the basolateral and apical membranes of the choroida epithelium, it should be possible to assess the role (and possible interaction) of two transport enzymes (carbonic anhydrase and Na-K-ATPase) at the blood-CSF barrier. Ontogenetic stides of the lareral and fourth ventricular choroid plexus will be carried out in order to correlate choroid epithelial epithelial cell ionic concetrations (after treatment with ouabain or acetazolamide) with enzyme (anhydrase and ATPase) activities in the tissue; to determine if organic acid drugs are actively transported by carrier systems which translocate inorganic anions; to ascertain the reationship between the distribution of bicarbonate and chloride in the choroid plexus-CSF system; and to investigate the interaction between hydrogen ion and potassium transport both into and out of the CSF sytem. Any improvement in the therapeutic procedures for correcting disorders such as hydrocephalus, deleterious effects of severe distortions of acid-base meta-bolism in the immature CNS, excessive penetration drugs into the fetal and neonatal brain, etc., is to a large extent dependent upon gaining a better appreciation of the basic physiology of the neyral barrier systems in the immature brain.