Using readily available labeled compounds, [4,5,6,8-(13)C4]guanine was synthesized in high overall yield. Intermediates as well as the final product were characterized by 1H NMR, 13C NMR, and high resolution mass spectrometry. The labeled guanine was used to generate [13C4]-labeled analogs of the guanine adducts, N2,3-ethenoguanine and 7-(2-hydroxyethyl)guanine. The application of such adducts in isotope dilution mass spectrometry was illustrated with DNA samples from rats exposed to two different mutagenic compounds, vinyl chloride and ethylene oxide.