The synthetic model ultimate carcinogen N-benzoyloxy-N-methyl-4-aminoazobenzene reacted in vitro with either calf thymus or rat liver DNA to yield approximately 1 bound residue per 1,000 nucleotides. The DNA was enzymatically hydrolyzed and subjected to high-pressure liquid chromatographic analysis which indicated the presence of at least 6 N-methyl-4-aminoazobenzene (MAB) adducts. Two of the products cochromatographed with MAB-DNA adducts formed in rat liver in vivo following oral administration of the precarcinogen MAB. These two adducts were identified by UV, mass, and nuclear magnetic resonance spectroscopy as N-(deoxyguanosin-8-yl)- and 3-(deoxyguanosin-N(2)-yl)-MAB: the first adduct was initially the predominant product in vivo, ut it could not be detected 7 days after treatment, and the second remained at a constant level for 14 days and therefore appeared to be a persistent lesion.