The carcinogens 4-aminobiphenyl, 2-aminofluorene and their N-acetyl derivatives form DNA adducts in vivo with the aryl nitrogen attached at C-8 of guanine. These adducts are proposed to arise through the reaction with the DNA base of a nitrenium ion obtained by N-0 heterolysis of a hydroxylamine ester obtained metabolically by oxidation and esterification. Routes have now been discovered for the photochemical generation of these nitrenium ions, the N-acetyl derivatives via a photoheterolysis reaction and the N-H derivatives by protonation of the singlet nitrene photochemically generated from the azide precursor. The nitrenium ions are observed as transient intermediates with laser flash photolysis, and the kinetics of their reaction with various nucleophiles directly monitored. These results, coupled with competition kinetics experiments, show that the nitrenium ions derived from the above amines and amides are relatively long-lived in water, with a remarkably high selectivity in water for reaction with 2'-deoxyguanosine and its 5'-phosphate. The C-8 adduct is the product of this reaction. These behaviours differ considerably from those of analogous carbenium ions. Reactions with vinyl ethers have also been investigated, and also show significant differences between the two types of electrophile.