Abstract Covalent binding of reactive metabolites of pentachloropheno (PCP) was investigated both in vitro andin vivo in the livers of male Sprague-Dawley rats via measurement of protein adducts. Cysteinyl adducts of quinones andsemiquinones in liver cytosolic (Cp) andnuclear (Np) proteins were assayed after catalytic cleavage by Raney nickel. Results from in vitro experiments confirmed that PCP metabolism produced tetrachlorobenzoquinones andthe comsponding tetrachlorobentosemiquinones which subsequently bound to sulphydryl groups in liver proteins. In vivo, the production of cysteinyl adducts increased with the administered dosage (0-40 mg PCP per kg body weight) andpresented evidence of saturable metabolism. Results suggest two metabolic pathways for PCP, including a high-affinity low-capacity pathway anda low-affinity high-capacity pathway. Time-course experiments in vivo andin vitro suggested that quinone adducts partlcipated in multiple substitution reactions with protein and/or non-protein thiols, andpointed to possible formation of protein-protein cross-links in vivo. The elimination rate constants of quinone adducts in vitro were about 0.35 h(-1) in liver Cp. The elimination of quinone adducts in vivo appeared to follow biphasic kinetics with rate constants for the terminal phase being 0.014 and0.008 h(-1) in liver Cp andNp, respectively.