Two examples are described of the use of NMR spectroscopy to study the modification of DNA structure by carcinogens. The reaction of ethylene dibromide involves initial conjugation with glutathione, catalysed by glutathione S-transferase. Reaction of this adduct with DNA occurs at N7 of guanine. Through the use of stereospecifically 1,2-dideuteriated ethylene dibromide, the mechanism of reaction has been shown to involve an odd number, i.e. three, of SN2 inversions. Correlation spectra (COSY) were employed to analyse reaction stereochemistry. The relative configuration of the deuterium atoms in the products was initially assigned by 1H nuclear Overhauser effect (NOE) difference spectra and then confirmed by an independent synthesis of stereospecifically dideuteriated glutathione-guanine adducts. The second example involves reaction of the epoxide of aflatoxin B1 with DNA to form covalent adducts at N7 of guanine. Adduct formation was found to enhance duplex stability. Chemical shift changes for aflatoxin protons in the covalent adduct when compared with those for aflatoxin B1 non-covalently associated with DNA suggest that covalently linked aflatoxin is intercalated. NOEs confirm that the aflatoxin moiety is intercalated and show that it is on the 5' side of the guanine. This geometry leads to d(ATCGAT)2 forming an adduct in which only one chain has been modified by aflatoxin, while d(ATGCAT)2 forms a complex in which both chains have been modified.