Omeprazole induces human CYP1A1 and CYP1A2 in human hepatoma cells and human liver. Aryl hydrocarbon receptor (AHR) is shown to be involved in this induction. However, its precise molecular mechanism remains unknown because the chemical activates AHR without its direct binding in contrast to typical AHR ligands such as 3-methylcholanthrene (3MC) and beta-naphthoflavone (BNF). Human CYP1A1 and CYP1A2 genes are located in a head-to-head orientation sharing about 23 kb 5'-flanking region. Recently, we succeeded to measure CYP1A1 and CYP1A2 transcriptional activities simultaneously using dual reporter gene constructs containing the 23 kb sequence. In this study, transient transfection assays have been performed using numbers of single and dual reporter constructs to identify omeprazole-responsive region for CYP1A1 and CYP1A2 induction. Reporter assays with deletion constructs have demonstrated that the omeprazole-induced expression of both CYP1A1 and CYP1A2 is mediated via the common regulatory region containing multiple AHR-binding motifs (the nucleotides from -464 to -1829 of human CYP1A1), which is identical with the region for BNF and 3MC induction. Interestingly, omeprazole activated the transcription of CYP1A1 and CYP1A2 to similar extents while BNF and 3MC preferred CYP1A1 expression. We have also found that primaquine is an omeprazole-like CYP1A inducer, while lansoprazole and albendazole are 3MC/BNF-like in terms of the CYP1A1/CYP1A2 preference. The present results suggest that omeprazole as well as BNF and 3MC activates both human CYP1A1 and CYP1A2 expression through the common regulatory region despite that omeprazole may involve a different cellular signal(s) from BNF and 3MC.