Inflammatory cells such as phagocytes, neutrophils, and macrophages have been implicated in the pathogenesis of several forms of clinical and experimental tumor development. It is hypothesized that this process is mediated by the production of reactive species including NO., O2.-, H2O2, and ONOO- which inflict DNA damage. In this study, the role of NO. in combination with oxygen radicals in DNA damage was investigated. DNA deamination (xanthine) and oxidation [5-(hydroxymethyl)uracil (5HMU), 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FAPY-G), and 8-oxoguanine (8oxoG)] products were identified in the DNA of macrophages (RAW264.7) activated with Escherichia coli lipopolysaccharide (LPS) and mouse gamma-interferon (INF-gamma). The formation of these products was inhibited by N-methyl-L-arginine (NMA), a nitric oxide synthase inhibitor. NMA inhibited only the production of nitric oxide and had no effect on superoxide production. These results demonstrate that NO. plays a dual role in damaging the DNA of activated macrophages. Autoxidation of NO. leads to nitrosating species which cause deamination of bases. Reaction of NO. with O2.- leads to DNA oxidative damage due to the formation of peroxynitrite which may have HO.-like oxidizing potential. Another possible mechanism of oxidative damage by NO. could be the mobilization of free iron by NO. which could ultimately cause Fenton-type reactions. Therefore, nitric oxide not only leads to deamination of DNA bases but is also an obligatory factor in oxidative damage to DNA.