The mechanisms underlying aryl hydrocarbon receptor (AHR) activation by agonists and circumstances that increase the sensitivity toward agonists and AHR inhibition by antagonists are diverse and still not fully understood. AHR antagonist, 2-methyl-2 H-pyrazole-3-carboxylic acid (2-methyl-4- o-tolylazo-phenyl)-amide, CH223191, has been reported to inhibit the AHR transcription activity. However, CH223191 antagonist activity toward an AHR endogenous ligand, 6-formylindolo[3,2- b]carbazole (FICZ), and its mode of action remain to be elusive. Male BALB/c albino mice, HepG2 cells, and HepG2-XRE-Luc carrying cytochrome P4501A1 (CYP1A1) gene linked to a luciferase reporter were exposed to FICZ alone or in combination with CH223191, buthionine-( S, R)-sulfoximine (BSO), and N-acetyl-l-cysteine (NAC) for 5 h. Microsomal and cellular CYP1A1 enzyme activities, cellular FICZ levels, CYP1A1 reporter activity, mitochondrial membrane potential, and mitochondrial-dependent reactive oxygen species (ROS) formation were measured. In this study, we showed that AHR activity induced by an AHR endogenous ligand, FICZ, in a dose-dependent manner could be suppressed by CH223191. Indeed, we observed that CH223191 is able to inhibit the catalytic activity of CYP1A1, with an IC50 value of 1.48 μM. Our experiments with silencing RNA sequences showed that ROS formation by mitochondria might take part as a primary event in the downregulation of CYP1A1 by CH223191. We describe a new mechanism for inhibition of AHR-induced CYP1A1 by CH223191. The sensitivity of the AHR to oxidants and its possible reversibility by antioxidants supports the view that CH223191-induced mitochondrial dysfunction might be involved in this pharmacological event.