Cyclopenta(c,d)pyrene (CPP) is a ubiquitous polycyclic aromatic hydrocarbon with potent mutagenic and carcinogenic activity. The trans isomer of 3,4-dihydro-3,4-dihydroxy-cyclopenta(c,d)pyrene has been shown to be the major metabolic product of CPP in rat, mouse, or human microsomal systems as well as in peroxyl radical-generating systems indicating the preferential formation of its obligatory precursor, CPP-3,4-epoxide. The direct mutagenicity of CPP-3,4-epoxide, the inactivity of 3,4-dihydro-CPP, and the DNA adduct forming capacity of CPP in vivo has prompted analysis of the DNA adduct forming capacity of CPP-3,4-epoxide to provide information pertaining to: (1) the role this postulated major ultimate mutagenic metabolite may plan in the formation of DNA adducts in vivo; (2) the base selectivity of CPP-3,4-epoxide DNA adducts; and (3) the role of CPP-3,4-epoxide in the mutagenicity/carcinogenicity of CPP. CPP-3,4-epoxide was reacted with calf thymus DNA, dGp, dAp, dTp, dCp, poly dB-dC, poly dA-dT, and poly dG. Adducts were analyzed by the butanol-enhanced version of (32)P-postlabeling. Four major and at least three minor adducts formed with DNA in vitro, which were further analyzed for their base-selectivity. A similar spectra of adducts was exhibited by dGp, poly dG-dC and poly dG. dCp, dTp, and dAp formed one, two, and four adducts, respectively. Adducts derived from either dGp, poly dG-dC, or poly dG comigrated with the DNA adducts in 3 solvent systems indicating that CPP-3,4-epoxide forms DNA adducts almost exclusively with deoxyguanosine.