The choroid plexuses arise from the ependymal lining of the brain ventricles and protrude into the ventricles. They are quite small structures and in the adult human weigh only 2 g in total (25). In the lateral ventricles of the mammalian brain, the choroid plexuses form sheet-like structures, whereas branched villus-like structures are found in the third and fourth ventricles (19). Each choroid plexus consists of a continuous monolayer of cuboidal to columnar epithelial cells situated on a core of connective tissue (FIGURE 1, B AND C). The part of the blood-CSF barrier formed by the choroid plexus consists of the fenestrated endothelium of the capillaries with their basal lamina, sparse connective tissue, a basement membrane, and the single layer of epithelial cells. The epithelium is regarded as the major barrier to the movement of substances between the blood and the CSF. The epithelial barrier is defined by the plasma membranes and lateral intercellular spaces with relatively leaky tight junctions. The ventricular surface is characterized by numerous microvilli and a central bundle of long motile cilia, as reviewed previously (86, 75). The basal surface is relatively smooth, but the lateral surfaces appear very irregular with many basal infoldings (44). The blood flow to the choroid plexuses is 3 ml·g1 ·min1 , which is 10 times higher than the flow to the brain parenchyma and 5 times higher than that to the kidney (15, 30).