AbstractBenzanthrone, an anthraquinone dye intermediate, is commonly used for the synthesis of a number of polycyclic vat and disperse dyes. Our prior studies have shown that benzanthrone can be metabolized by rat hepatic microsomal cytochrome P450 (P450) (Biochem. Int., 18, 1989, 1237). In this study, the interaction of benzanthrone with rat hepatic microsomal P‐450 and its effect on xenobiotic metabolism have been investigated. Parenteral administration of benzanthrone (40 mg/kg body weight) for 3, 7, or 21 days caused no change in the relative body weight or organ weight of rats. The levels of P450 were found to be reduced (33%‐50%) in all the benzanthrone‐exposed animals at all the time periods. In vitro addition of benzanthrone caused a spectral change with oxidized P450 and concentration‐dependent reduction in the carbon monoxide spectrum of dithionite‐reduced P450. The addition of benzanthrone to hepatic microsomes prepared from phenobarbital‐treated rats resulted in spectral changes characterized by an absorbance maximum at 397 nm indicative of type I binding. In vitro addition of benzanthrone showed a concentration‐dependent inhibition of hepatic aminopyrineN‐demethylase (APD) and ethoxyresorufin‐O‐deethylase (ERD) activities with respective I50values of 9.5 × 10−4and 8.0 × 10−5M. However, the inhibition of aryl hydrocarbon hydroxylase (AHH) even at the highest concentration of benzanthrone (10−2M), was of the order of only 29%. In vivo administration of benzanthrone also led to the inhibition of APD, AHH, and ERD activities at all treatment times although the magnitude of inhibition was of a lower order. Benzanthrone treatments caused significant depletion of glutathione content with a concomitant enhancement of lipid peroxidation. Also, benzanthrone resulted in a significant inhibition of the cytosolic enzyme, GSH‐transferase (23%–34%), although the other enzyme, quinone reductase, which helps in the removal of toxic quinones, was found to be elevated (184%–199%). These results suggest several mechanisms by which benzanthrone may inhibit P450. This impairment of xenobiotic metabolism as well as the enhancement of lipid peroxidation may be relevant to the observed symptoms of be