7,8-Dihydrobenzo[a]pyrene (7,8-H2BaP) was metabolized by rat liver microsomes to form 7,8,9,10-tetrahydro-BaP trans-9,10-diol, 7,8,9,10-tetrahydro-BaP cis-9,10-diol, 7-hydroxy-7,8-H2BaP, 8-hydroxy-7,8-H2BaP, two phenolic products of 7,8-H2BaP [abbreviated as 7,8-H2BaP phenol 1 and phenol 2 according to their elution order on reversed-phase high-performance liquid chromatography (HPLC)], 4,5,7,8-tetrahydro-BaP trans-4,5-diol, BaP cis-7,8-dihydrodiol, BaP, and the metabolites known to be formed from the metabolism of BaP. Metabolites were isolated by reversed-phase and normal-phase HPLC and identified by ultraviolet-visible absorption and mass spectral analyses and by comparing their retention times with synthetic standards whenever available. The enantiomeric compositions of some mono-ol and diol metabolites were determined by chiral stationary phase HPLC. The optical purities of monool and diol metabolites formed were found to be dependent on the nature of cytochrome P-450 isozymes present in liver microsomes. Metabolites formed by liver microsomes from untreated, phenobarbital-treated, 3-methylcholanthrene-treated, and polychlorinated biphenyls (Aroclor 1254)-treated male Sprague-Dawley rats were quantified by using specifically tritium-labeled [10-3H]-7,8-H2BaP and liquid scintillation counting of fractions collected from reversed-phase HPLC. A portion (2-7% depending on the type of microsomes used) of the BaP found was formed nonenzymatically in microsomal metabolism of 7,8-H2BaP. The formations of other major metabolites were all cytochrome P-450 isozymes dependent since their formations were inhibited by carbon monoxide and were dependent on the presence of reduced nicotinamide adenine dinucleotide phosphate. Furthermore, the formations of tetrahydrodiols, monools, and phenols were not inhibited by the epoxide hydrolase inhibitor, 3,3,3-trichloropropylene 1,2-oxide. The relative mutagenic activities toward Salmonella typhimurium TA98 at 2 nmol of chemical per plate and 10 microliters of liver S9 fraction were: (+/-)BaP trans-7,8-dihydrodiol approximately equal to 7,8-H2BaP approximately equal to 7,8-H2BaP phenol 2 greater than (+/-)Bap cis-7,8-dihydrodiol greater than BaP approximately equal to 8-hydroxy-7,8-H2BaP greater than 7,8-H2BaP phenol 1 greater than 7-hydroxy-7,8-H2BaP. The results suggest that, in addition to the bay region 7,8,9,10-tetrahydro-BaP 9,10-epoxide, metabolic products formed by hydroxylations at the aliphatic and aromatic carbons of 7,8-H2BaP and their subsequent metabolism at the 9,10-double bond may also contribute to the carcinogenic activities of 7,8-H2BaP.