5-Methylcbrysene (5-MeC) can form two bay region dihydrodiol epoxides: 1,2-dihydroxy-3-4-epoxy-1,2,3,4-tetrahydro-5-methylchrysene (DE-I) which has the methyl group and the epoxide ring in the same bay region, and 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydro-5-methylchrysene (DE-II). In a previous study, we observed that the ratio of DE-I:DNA adduds to DE-II:DNA adducts in mouse epidermis, 24 h after application of [3H]5-MeC metabolites was 2.7 to 1. To investigate the basis for this observation we have now studied: (i) the formation of [3H]5-MeC in mouse epidermis in vivo at various time intervals from 0.33 to 24 h; (II) the persistence of DE-I:DNA adducts and DE-II:DNA adducts in mouse epidermis at 4-48 h after application of [3H]5-MeC and (iii) the reactions of DE-I and DE-II with calf thymus DNA in vitro. In contrast to results obtained with mouse liver 9000 g supernatant, the dihydrodiol precursors of DE-I and DE-II were present in equivalent quantities in mouse epidermis in vivo at every time point studied. The ratio of DE-I:DNA adducts to DE-II:DNA adducts in mouse epidermis was constant throughout the time period studied. However, the extent of formation of DE-I:DNA adducts was greater than that of DE-II:DNA adducts upon reaction of DE-I or DE-II with calf thymus DNA in vitro. These results suggest that differences in reactivity with DNA of DE-I and DE-II may be responsible for the higher levels in mouse epidermis of DE-I:DNA adducts compared with DE-II:DNA adducts and provide a possible basis for the observed enhancing effect of a bay region methyl group on the carcinogenicity of polynuclear aromatic hydrocarbons.