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The differentiating choroid plexus in rats at ages ranging from prenatal day 16 to postnatal day 3 was investigated with a special emphasis on the immunolocalization of Na+.K+ATPase in the choroid epithelium and neocapillarization of the choroid stroma. Immunoreactions for Na+.K+ATPase were localized on the apical plasma membrane of the simple choroid epithelium throughout the stages examined, but it also appeared on the basolateral plasma membrane till prenatal day 18, prior to the formation of the tight junctions as seen from both a marker experiment using lanthanum chloride and immunoreactions for tight junction-associated protein (ZO-1). This suggests that a polarity of the enzyme localization to the apical plasma membrane may take place after the establishment of the blood-cerebrospinal fluid barrier. Mesenchymal cells in the choroid stroma immunoreacted to the laminin antibody and occasionally contacted each other--forming solid cell cords--or endothelial cells of the preexisting capillaries. The contact areas immunoreacted to this antibody. Since there were few or no mitotic figures of the endothelial cells, the involvement of laminin in a mechanical linkage between the adjacent vasoformative mesenchymal cells as well as the endothelial lining in a manner suggestive of vasculogenesis was indicated.