Cytochrome P450 1A2 (CYP1A2) is an inducible hepatic protein which binds dioxins and produces a dose-dependent hepatic sequestration. Dioxins bind to the Ah receptor (AhR), which regulates a variety of effects including the expression of CYP1A2. There is a direct concordance between the ability to bind AhR and the toxic potency of each chemical. Polyhalogenated aromatic hydrocarbons (PHAHs) have been classified as dioxin-like or nondioxin-like based on their ability to bind the aryl-hydrocarbon receptor (AhR), induce toxicity, and bioaccumulate. 90-day subchronic mouse studies have shown dose-dependent increases in liver/fat concentrations of several dioxin-like chemicals, supporting the presence of an inducible hepatic binding protein1 . Our laboratory demonstrated the importance of CYP1A2 as the specific hepatic binding protein responsible for hepatic sequestration of TCDD and dioxin-like compounds by exposing CYP1A2 knockout mice and parental strains to TCDD, 4-PeCDF (a dioxin-like PHAH), and PCB 153 (a nondioxin-like PCB) 2 . The ability of the liver to sequester TCDD and 4- PeCDF was significantly decreased in the knockouts resulting in increased concentrations of these chemicals in extrahepatic tissues, while the distribution of PCB 153 was unaltered in the knockout mice. These studies demonstrate the influence of CYP1A2 on the distribution of dioxins in rodents.