Ubiquitous environmental agents [e.g., polynuclear aromatic hydrocarbons (PAHs) and their nitrated derivatives (NO2-PAHs)] that are known to induce mammary cancer in rodents are regarded as potential human risk factors for inducing analogous human cancers. Although 6-nitrochrysene (6-NC) is less abundant than other NO2-PAHs in the environment, it is the most potent mammary carcinogen in the rat; its carcinogenic potency is not only higher than that of the carcinogenic PAH, benzo[a]pyrene (B[a]P), but also of the well-known carcinogenic heterocylic aromatic amine, 2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine (PhIP). Studies in rats and in vitro assays have indicated that 6-NC can be activated by simple nitroreduction leading to the formation of 6-hydroxylaminochrysene (N-OH-6-AC); this metabolite yielded N-(deoxyguanosin-8-yl)-6-aminochrysene (N-[dG-8-yl]-6-AC) and 5-(deoxyguanosin-N2-yl)-6-aminochrysene (5-[dG-N2-yl]-6- AC. These lesions are likely to cause mutations if they are not removed by cellular defense mechanisms before DNA replication occurs. However, nothing is known about the susceptibility of these adducts to nucleotide excision repair (NER), the major cellular repair system that removes bulky adducts. In order to address this issue, we synthesized the N-(dG-8-yl)-6-AC and 5-(dG-N 2-yl)-6-AC lesions and site-specifically inserted these lesions into 135-mer DNA duplexes. These constructs were incubated with NER-competent nuclear extracts from human HeLa cells. The efficiency of repair of these lesions was ∼8 times less efficient than that in the case of the well-known and excellent substrate of NER, the intrastrand cross-linked cis- diaminodichloroplatinum II adduct in double-stranded DNA (cis-Pt), but similar to N2-dG adducts derived from the (+)-bay region diol epoxide of B[a]P [(+)-trans-B[a]P-N2-dG]. The results support the hypothesis that the N-(dG-8-yl)-6-AC and 5-(dG-N2-yl)-6-AC lesions may be slowly repaired and thus persistent in mammalian tissue which could, in part, account for the potent tumorigenic activity of 6-NC in the rat mammary gland.