We have reported previously that cyclopenteno[c,d]pyrene (CPP), an environmentally prevalent polycyclic aromatic hydrocarbon, is activated as a bacterial mutagen by several model systems which generate peroxyl radicals. In this report we present our findings on the chemical fate of CPP in these activating systems. The peroxyl radical systems employed are microsomal prostaglandin H synthase and arachidonic acid, the hematin-catalyzed decomposition of a lipid hydroperoxide, and the autoxidation of the sulfite anion. Reverse-phase HPLC analysis of stable products of CPP metabolism yielded qualitatively identical profiles from the first two systems. The three major products from these systems were analyzed by UV/visible and fluorescence spectroscopy, and a mass spectrum was obtained for the most abundant product. Based on these spectral analyses and on chromatographic behavior, the three products were identified as the cis- and trans-isomers of 3,4-dihydroxy-3,4-dihydro-CPP and 4-keto-(3H)-CPP. The identities of these products and their quantitative distributions relative to the epoxide hydrolase activities present in the microsomal system and the hematin system clearly establish 3,4-epoxy-CPP as the key intermediate and probable active mutagen generated in the peroxyl radical-dependent metabolism of CPP. This epoxidation of the activated aliphatic double bond of CPP extends the known range of peroxyl radical-dependent oxygenations by demonstrating the direct, one-step activation of a carcinogenic, environmentally relevant hydrocarbon. Strikingly different results are obtained in the sulfite-dependent system. The epoxide-derived metabolites seen with the peroxyl radical systems are very minor products. Instead, two product peaks elute near the solvent fron on reverse-phase HPLC. These are apparently monohydroxy-CPP sulfonates. Such products may form either by the direct addition of the sulfite anion radical to the activated double bond of CPP or by peroxyl radical-dependent epoxidation of CPP followed by nucleophilic addition of sulfite. Precedent for both of these reactions has been reported with analogous benzo[a]pyrene derivatives. The occurrence of these radical-dependent transformations in intact mammalian systems has not been investigated, but the ability of all three model systems employed to convert CPP to potent bacterial mutagens implies that these pathways should be studied further.