Metabolites of (+/-)-trans 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene formed by a rat liver microsomes and by a highly purified monoxygenase system were analyzed by high-pressure liquid chromatography. Four stereoisomeric tetraols of 7,8,9,10-tetrahydrobenzo[a]pyrene, known solvolysis products of the two highly mutagenic stereoisomers of the 9,10-epoxide of the 7,8-dihydrodiol, were identified as products. The ratio of the two highly unstable diol epoxides formed (7 beta,8alpha-dihydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, diol epoxide 1; 7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, diol epoxide 2) ranged from about 1.7 to 0.4. The diol epoxides are sufficiently reactive to alkylate phosphate buffer (pH 7.4) at 37 degrees. Microsomes, particularly those from control animals, formed a substantial amount of an additional metabolite that appears to be phenolic. In analogy to benzo[a]pyrene, the metabolism of the 7,8-dihydrodiol shows similar induction after pretreatment of rats with phenobarbital or 3-methylcholanthrene. Neither diol epoxide appears to be a substrate for epoxide hydrase based on the ratis of tetraols formed in the presence or absence of epoxide hydrase. In view of the known carcinogenicity of benzo[a]pyrene 7,8-oxide and 7,8-dihydrodiol and of the marked mutagenicity of the stereoisomeric diol epoxides, both of these diol epoxides qualify for consideration as "ultimate carcinogen(s)" of benzo[a]pyrene.