P450s belonging to the CYP3A subfamily constitute major forms in livers and intestines of human and experimental animals. Expressions of CYP3A forms are induced by treatment with many chemicals, but their profiles differ among humans and experimental animals. Molecular mechanisms of CYP3A gene activation by chemicals had been little known until recently. A novel steroid hormone receptor, PXR (pregnane X receptor), has been isolated from mouse, rat, rabbit and human as a signal transduction molecule in CYP3A gene transactivation. Species differences observed on the induction profiles might be explained by the species specific response of their PXRs to chemicals. However, the diverse induction profiles of CYP3A forms are possible to be caused by interaction of PXR with transcription factors other than RXRα. In support with this idea, we have found that rat CYP3A1 gene is transactivated by an orphan nuclear receptor, ARP-1 with PXR in the presence of inducers. To assess the induction potency of various chemicals for human CYP3A forms, further researches are necessary to clarify the transcriptional regulation of CYP3A forms.