Aryl hydrocarbon receptor (AhR) ligands, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and metals, typified by arsenite (As(III)), are environmental cocontaminants, and their molecular interaction may disrupt the coordinated regulation of the carcinogen activating enzyme cytochrome P450 1A1 (CYP1A1). Therefore, in this study we examined the effects of coexposure to As(III) and TCDD on the expression of CYP1A1 in HepG2 cells. Our results showed that As(III) caused a dose-dependent decrease in TCDD-mediated induction of CYP1A1 mRNA, protein, and catalytic activity levels. As(III) significantly inhibited TCDD-mediated induction of AhR-dependent luciferase reporter gene expression without altering CYP1A1 mRNA stability. In addition, As(III) increased heme oxygenase-1 (HO-1) mRNA, which coincided with a further decrease in the CYP1A1 catalytic activity levels. When a competitive HO-1 inhibitor, tin mesoporphyrin, was applied to HepG2 cells or the cells were transfected with siRNA for HO-1 there was a partial restoration of the inhibition of TCDD-mediated induction of CYP1A1 catalytic activity. Treatment of cells with heme or hemoglobin partially restored the As(III)-mediated inhibition of CYP1A1 catalytic activity. On the other hand, cobalt protoporphyrin increased HO-1 mRNA, with a concomitant decrease in CYP1A1 activity, without affecting CYP1A1 mRNA, which was reversed by HO-1 siRNA transfection. This study demonstrates that As(III) down-regulates CYP1A1 through transcriptional and posttranslational mechanisms. In addition, HO-1 is involved in the As(III)-mediated down-regulation of CYP1A1 at the catalytic activity level.