AhR, a ligand-activated transcription factor, mediates xenobiotic signaling to enhance the expression of target genes, including drug-metabolizing cytochrome P450s. The recent development of serveral new techniques, including chromatin immunoprecipitation and RNA interference, has expanded and deepened our knowledge of AhR funciton in the xenobiotic signal transduction. In this review, we briefly summarize our current understanding of the activation and inactivation of AhR activities and discuss the future directions of AhR research. (c) 2005 Elsevier Inc. All rights reserved.