To explain the biological activity of 6-nitrobenzo[a]pyrene (6-nitroBaP), male Sprague-Dawley rats were induced with 3-methylcholanthrene. Liver microsomes were incubated with magnesium chloride, an NADPH generating system and 6-nitroBaP in acetone. The mixture was chilled under oxygen-free argon gas and protein was precipitated with an equal volume of cold methanol containing triethylamine. Protein was further precipitated with zinc and sodium sulfate and centrifuged. Both the sediment and the supernatant were extracted with benzene and ethyl acetate. The organic extract was washed with water, 2% sodium hydroxide solution, water and then dried with anhydrous sodium sulfate. Solvents were removed and the residue was chromatographed on silica gel plates with hexane containing increasing amounts of benzene. The UV and mass spectra of products were examined. Liver microsomal metabolites of 6-nitroBaP consisted of 7,8- and 9,10-dihydrodiols and also benzo[a]pyrene (BaP) and BaP-quinones. cis-Forms of 6-nitroBaP-7,8- and -9,10-dihydrodiols were synthesized.