One-electron oxidation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG) yielded a guanidinohydantoin derivative (dGh) and a spiroiminodihydantoin derivative (dSp), both putatively mutagenic products that may be formed in vivo. The nucleoside dGh was the major product at room temperature, regardless of pH. The results are contrary to previously published model studies using 2',3',5'-triacetoxy-8-oxo-7,8-dihydroguanosine (Luo, W.; Miller, J. G.; Rachlin, E. M.; Burrows, C. J. Org. Lett. 2000, 2, 613; Luo, W.; Miller, J.G.; Rachlin, E.M.; Burrows, C.J. Chem. Res. Toxicol. 2001, 14, 927), who observed a spiroiminodihydantoin derivative as the major product at neutral pH. Clearly, the functional groups attached to the ribose moiety of 8-oxodG influence the oxidation chemistry of the nucleobase derivative. To explore this chemistry in vivo, (14)C-labeled 8-oxodG was synthesized and incubated with growing MCF-7 human breast cancer cells, resulting in the incorporation of the compound into cellular DNA as measured by a novel accelerator mass spectrometry assay.