Abstract The aryl-hydrocarbon receptor (AhR) is a basic helix-loop-helix/Per-Arnt-Sim transcription factor that can be activated by exogenous as well as endogenous ligands. AhR is traditionally associated with xenobiotic metabolism. In an attempt to identify novel target genes, C57BL/6J mice were treated with β-naphthoflavone (BNF), a known AhR ligand, and genome-wide expression analysis studies were performed using high-density microarrays. Constitutive androstane receptor (CAR) was found to be one of the differentially regulated genes. Real-time quantitative polymerase chain reaction (qPCR) verified the increase in CAR messenger RNA (mRNA) level. BNF treatment did not increase CAR mRNA in AhR-null mice. Time-course studies in mice revealed that the regulation of CAR mRNA mimicked that of Cyp1A1 , a known AhR target gene. To demonstrate that the increase in CAR mRNA translates to an increase in functional CAR protein, mice were sequentially treated with BNF (6 hours) followed by the selective CAR agonist, TCPOBOP (3 hours). qPCR revealed an increase in the mRNA level of Cyp2b10 , previously known to be regulated by CAR. This also suggests that CAR protein is present in limiting amounts with respect to its transactivation ability. Finally, CAR was also up-regulated in primary human hepatocytes in response to AhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene. Conclusion: This study identifies a mode of up-regulating CAR and potentially expands the role of AhR in drug metabolism. This study also demonstrates in vivo up-regulation of CAR through chemical exposure. (HEPATOLOGY 2007;46:209–218.)