The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 1) is a potent lung carcinogen in laboratory animals and is believed to play a key role in the development of lung cancer in smokers. Metabolic activation of NNK leads to the formation of pyridyloxobutyl DNA adducts, a critical step in its mechanism of carcinogenesis. In addition to DNA nucleobase adducts, DNA phosphate adducts can be formed by pyridyloxobutylation of the oxygen atoms of the internucleotidic phosphodiester linkages. We report the use of a liquid chromatography-nanoelectrospray ionization-high-resolution tandem mass spectrometry technique to characterize 30 novel pyridyloxobutyl DNA phosphate adducts in calf thymus DNA (CT-DNA) treated with 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc, 2), a regiochemically activated form of NNK. A (15)N3-labeled internal standard was synthesized for one of the most abundant phosphate adducts, dCp[4-oxo-4-(3-pyridyl)butyl]dC (CpopC), and this standard was used to quantify CpopC and to estimate the levels of other adducts in the NNKOAc-treated CT-DNA. Formation of DNA phosphate adducts by NNK in vivo was further investigated in rats treated with NNK acutely (0.1 mmol/kg once daily for 4 days by subcutaneous injection) and chronically (5 ppm in drinking water for 10, 30, 50, and 70 weeks). This study provides the first comprehensive structural identification and quantitation of a panel of DNA phosphate adducts of a structurally complex carcinogen and chemical support for future mechanistic studies of tobacco carcinogenesis in humans.