The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1- butanone (NNK) is a potent pulmonary carcinogen in rats and is believed to be one cause of lung cancer in smokers. NNK is metabolized to 4-(methylnitrosamino) -1-(3-pyridyl)-1-butanol (NNAL), which is also a strong lung carcinogen in rats and has a chiral center at its 1-carbon. Previous studies have demonstrated that cytochrome P450-catalyzed α-hydroxylation of NNK in the lung leading to the formation of methyl and pyridyloxobutyl (POB)-DNA adducts is critical for its carcinogenicity. α-Hydroxylation of NNAL would similarly produce pyridylhydroxybutyl (PHB)-DNA adducts, but these have not been previously investigated in vivo. POB- and PHB-DNA adduct levels can indicate the amounts of pyridyloxobutylating and pyridylhydroxybutylating agents present in tissues of NNK- or NNAL-treated rats at any given point. Therefore, in this study, we developed a sensitive and quantitative liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring method to determine levels of the PHB-DNA adducts O6-[4-(3-pyridyl)-4- hydroxybut-1-yl]-2′-deoxyguanosine (O6-PHB-dGuo, 10b), O 2-[4-(3-pyridyl)-4-hydroxybut-1-yl]thymidine (O2-PHB-dThd, 11b), and 7-[4-(3-pyridyl)-4-hydroxybut-1-yl]-2′-deoxyguanosine (7-PHB-dGuo, 12b), the latter as the corresponding base 7-[4-(3-pyridyl)-4- hydroxybut-1-yl]-Gua (7-PHB-Gua, 14b) in DNA isolated from liver and lung of rats treated with 10 ppm NNK, (S)-NNAL, or (R)-NNAL in the drinking water for 20 weeks and sacrificed at 1, 2, 5, 10, 16, and 20 weeks. PHB-DNA adduct levels were higher in lung than in liver at each time point, consistent with previous studies of POB-DNA adducts in rats treated with NNK and NNAL in the drinking water. The results showed that NNK and (S)-NNAL behaved in a similar fashion, while (R)-NNAL was strikingly different. In the rats treated with NNK or (S)-NNAL, levels of each adduct at each time point were remarkably similar in lung, and levels of O2-PHB-dThd were generally greater than 7-PHB-Gua > O6-PHB-dGuo. The highest PHB-DNA adduct levels were found in lung and liver of rats treated with (R)-NNAL, suggesting that there are cytochrome P450s that efficiently catalyze the α-methyl hydroxylation of this compound. The results of this study provide further support for our hypothesis that (S)-NNAL is rapidly formed from NNK, sequestered at an unknown site in the lung, and then released and reoxidized to NNK with consequent DNA adduct formation resulting in lung carcinogenicity.