The multidrug resistance (mdr1) gene family encodes the efflux transporter P-glycoprotein (P-gp) which contributes to the functionality of the blood-brain barrier. We have characterised the regional expression of mdr1 genes in nai;ve rat brain. Adult male Sprague-Dawley rats (n=6) were sacrificed and their brains rapidly removed. Seven distinct anatomical regions were isolated by microdissection and the expression of mdr1a and mdr1b determined by quantitative reverse-transcriptase polymerase chain reaction. The mdr1a isoform was expressed in all brain regions investigated, while mdr1b was expressed to a quantifiable degree in hippocampus alone. These findings reveal a differential expression of mdr1 genes in normal rodent brain tissue and suggest that P-gp may afford a broader protection of the hippocampus than other brain structures.