Deoxyguanosine 3'-monophosphate (dGMP) was alkylated at the 7-position by dimethyl sulfate, ethylene oxide and styrene oxide in aqueous media and glacial acetic acid, respectively, to yield reasonable quantities of the products, which were purified by HPLC. dGMP adducts are needed as standards for the 32P-postlabelling assay. The stability of the adducts was studied at 37 degrees and neutral pH. The half-lives of disappearance of 7-methyl-dGMP and the beta-isomers of the styrene oxide adducts were about 250 min; 7-hydroxy-ethyl-dGMP and the alpha-isomers of the styrene oxide adducts had respective half-lives of 340 and 440 min. In all cases the main degradation product was the corresponding guanine adduct. The results demonstrate considerable lability of the 7-alkylation products of dGMP which has to be taken into consideration in devising the 32P-postlabelling assay.