The role of prostaglandin H (PGH) synthase and peroxyl radicals as well as cytochrome P-450 in the metabolism of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) was examined in fresh skin keratinocytes isolated from hairless mice. Labeled (+)-BP-7,8-diol was oxidized after incubation with the keratinocytes to syn- and anti-diolepoxides in greater than a 4:1 ratio as estimated by h.p.l.c. analysis of the stable hydrolysis products. Formation of diolepoxides was dependent on cell number and the concentration of BP-7,8-diol. Incubation in the presence of the PGH synthase substrate, 20:4 or the inhibitor, indomethacin did not alter the total formation or the ratio of diolepoxides. However, the addition of butylated hydroxyanisole (1 micron) an inhibitor of peroxyl radical dependent-metabolism significantly inhibited diolepoxide formation. The time course for the formation of the anti-diolepoxide and lipid peroxidation, measured as malondialdehyde was determined. The results suggest an excellent correlation between peroxyl radical and diolepoxide formation. Pretreatment of mice with the cytochrome P-450 inducer, beta-naphthoflavone greatly altered the metabolism of (+)-BP-7,8-diol by keratinocytes. The major metabolite was the syn-diolepoxide with significant formation of two unknown metabolites. Pretreatment of mice with BP-7,8-diol did not induce aryl hydrocarbon hydroxylase activity but did increase the yield of syn-diolepoxide formed from labeled (+)-BP-7,8-diol by 1.5-fold. Our results suggest that peroxyl radical-mediated metabolism is primarily responsible for the oxidation of (+)-BP-7,8-diol in control animals while the cytochrome P-450 system is primarily responsible for oxidation in animals pretreated with inducers.