Abstract: Polychlorinated biphenyls (PCBs) fall into the category of halogenated aromatic compounds (HACs) or dioxin-like compounds (DLCs) and are highly lipophilic environmental contaminants. Metabolism of DLCs generally involves monooxygenase enzymes of the CYP 1A family in which CYP 1A2 specifically exists in hepatocytes and is mainly responsible for much more toxic reactions of xenobiotics. In this work, we have examined the role of numerous PCBs as inhibitors of the demethylation of 7-methoxyresorufin, a process that is selectively catalyzed by CYP1A2. 2,3,7,8-Tetrachlorobenzo-p-dioxin (TCDD), 3,3’,4,4’-tetrachlorobiphenyl (PCB 77) and 3,3’,4,4’,5-pentachlorobiphenyl (PCB 126) were found to be the strongest inhibitors of methoxyresorufin-O-demethylase （MROD） activity, thus accounting for the high ability of hepatic tissue to selectively sequester these compounds.