P-Glycoprotein the multidrug resistance (mdr) efflux transporter is encoded by class 1 mdr genes (mdr1) in humans and rodent species. In rat liver and in rat hepatocytes in primary culture, expression of mdr1 genes can be induced with the carcinogenic aromatic amine 2-acetylaminofluorene (2-AAF). As a consequence, increased P-glycoprotein levels led to an accelerated efflux of vinblastine from the hepatocytes and to resistance towards vinblastine-mediated cytotoxicity. N-Hydroxylation, an obligatory initial step in the activation of 2-AAF into electrophilic DNA-binding metabolites is catalyzed predominantly by cytochrome P450 (CYP)1A2, an isozyme present in normal rat liver. In rat hepatocytes in primary culture, mdr1 induction with 2-AAF could be inhibited by addition of the CYP1A-inhibitor alpha-naphthoflavone, indicating the requirement for metabolic conversion of 2-AAF to act as an inducer of mdr1 gene expression. Both N-hydroxy-2-AAF and the mutagenic 2-AAF derivative N-acetoxy-2-AAF (AAAF) were more potent than 2-AAF as mdr1 inducers. mdr1 induction also decreased when deacetylation of AAAF, which strongly accelerates its conversion into a mutagen, was inhibited with paraoxon. Furthermore, rat liver epithelial cells stably transfected with mouse CYP1A2 showed inducibility of mdr1 gene expression with 2-AAF, whereas the parental cell line, which is devoid of CYP1A2 activity, did not. These findings indicate that electrophilic metabolites formed during 2-AAF or AAAF metabolism are responsible for mdr1 induction in rat hepatocytes. The increased mdr1 gene expression may reflect an adaptive cellular response to electrophiles which includes enhanced synthesis of P-glycoprotein aimed to protect the cell from further damage.