Extensive methylation was found upon reaction of N-methyl-N-nitrosourea with dATP. The products of this reaction were purified by repeated anion exchange column chromatography and were found to consist of adenine deoxyribonucleotides methylated on the base moiety, terminal phosphate, or both. Products methylated on the adenine ring were identified by co-migration of acid-hydrolyzed samples with authentic standards on reverse phase high pressure liquid chromatography. Products methylated on the sugar phosphate moiety were identified by digestion with snake venom phosphodiesterase, alkaline phosphatase, or both followed by polyethyleneimine cellulose thin layer chromatography. The results demonstrate production of gamma-phosphate-methyldATP, beta-phosphate-methyl-dADP, 1-methyldATP, and gamma-phosphate-methyl-1-methyldATP as well as the relatively unstable products 3-methyldATP and gamma-phosphate-methyl-3-methyl-dATP. The identities and amounts of products formed during this reaction in vitro are consistent with our finding that cellular deoxyribonucleotide pools are a significant target for N-methyl-N-nitrosourea (Topal, M. D., and Baker, M. S. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2211-2215).