Multidrug resistance-associated protein (MRP) actively transports a broad range of anionic compounds out of the cell. To date, six different homologues of MRP (i.e. MRP1-MRP6) have been identified. The current study examines the expression of the various MRP homologues in both primary cultured bovine brain microvessel endothelial cells (BBMEC) and the capillary-enriched fraction from bovine brain homogenates. RT-PCR analysis demonstrated the presence of MRP1, MRP4, MRP5 and MRP6 in both BBMEC and the capillary-enriched fractions of brain homogenates. While low levels of MRP3 were detected in the BBMEC, it was not observed in the capillary-enriched fraction. In addition, RT-PCR and Western blot studies indicated an absence of MRP2 expression in both blood-brain barrier preparations. The presence of several different MRP homologues in the brain microvessel endothelial cells may be important in controlling the permeability of the blood-brain barrier to organic anions.