The cells of the ependymal layer bordering the ventricles are not bound together by tight junctions and the layer is thought to be permeable to small solutes and proteins [17–20]. However, diffusion in the parenchyma is too slow to transfer material more than several hundred microns within 1–2 h2 (see e.g. [17, 21–25]). Thus normally neither transfer across the ependyma nor flow of CSF provides a rapid route for substances to reach the choroid plexuses from most of the parenchyma. For this reason, other than as the primary source of CSF, the choroid plexuses do not feature prominently in this review, which is concerned primarily with elimination of substances from the parenchyma.3 Readers interested in transporters at the choroid plexuses and the transport they mediate are well served by other reviews [2–4, 20, 26–38].