The reactivity of peroxynitrite (OONO-) with DNA is of interest because it is is released during chronic inflammation, a major contributor to cancer. Peroxynitrite was found to undergo homolytic addition to the C4-C5 double bond of 2'-deoxyguanosine with formation of 4,5-dihydro-5-hydroxy-4-(nitrosooxy)-2'-deoxyguanosine (nox-dG). This adduct may be a useful marker in the study of the chemical processes associated with the mutagenicity of peroxynitrite.