P-Glycoprotein is a plasma membrane drug efflux protein implicated in extrusion of cytotoxic compounds out of a cell. There is now evidence that suggests expression of this transporter at several subcellular sites, including the nucleus, mitochondria, and Golgi apparatus. This study investigated the localization and expression of P-glycoprotein at the nuclear membrane of rat brain microvessel endothelial (RBE4) and microglial (MLS-9) cell lines. Immunocytochemistry at the light and electron microscope levels using P-glycoprotein monoclonals antibodies demonstrated the localization of the protein at the nuclear envelope of RBE4 and MLS-9 cells. Western blot analysis revealed a single band of 170-kDa in purified nuclear membranes prepared from isolated nuclei of RBE4 and MLS-9 cells. These findings indicate that P-glycoprotein is expressed at the nuclear envelope of rat brain cells and suggest a role in multidrug resistance at this subcellular site. (c) 2007 Elsevier Inc. All rights reserved.