The choroid plexus traditionally has been considered the major, but not sole, component of the blood-CSF barrier (BCFB). Most analyses of the BCFB have been directed to the choroid plexuses, rather than to the arachnoid, because of the predominant function of the former in CSF secretion and homeostasis, The physiological literature on the choroid plexus (CP) is much more extensive than on the arachnoid membrane; consequently this review emphasizes methodologies employed to evaluate choroidal function.