P-Glycoprotein (PGP), a product of the multidrug resistance gene (mdr), acts as an adenosine triphosphate-dependent drug efflux system in cells. Initially, PGP was found in cancer cells, but it is now known that PGP is richly distributed in the adult brain. Passage to the central nervous system is limited by the blood-brain barrier (BBB), and mdr1 gene-deficient mice showed up-regulation of BBB permeability. In this study, we examined the expression and localization of PGP in the rat brain during development. PGP protein was predominantly detected in the membrane fraction of the adult rat brain, although it was also faintly detected in the cytosolic fraction. PGP protein in the membrane fraction was undetectable in the embryo and early stages of postnatal development by immunoblotting studies, was first detected on postnatal day (P) 7, and then gradually increased to reach a plateau. Such changes were observed commonly in the cerebral cortex, hippocampus, and cerebellum. Immunohistochemical studies showed that PGP immunoreactivity was first detected on P7, and intense PGP immunoreactivity was observed in the adult rat brain. Double-immunolabeling studies revealed that PGP was colocalized with von Willebrand factor-immunoreactive capillaries. We further examined the colocalization of PGP and astrocytes using glial fibrillary acidic protein (GFAP) as a marker. Three-dimensional analysis showed that the GFAP-immunoreactive astrocytes possessed fine processes which ensheathed capillaries, but the PGP immunoreactivity did not colocalize with the GFAP immunoreactivity. These results indicate that PGP expression increased with postnatal development and is localized in the brain capillaries.