Bioassays and DNA-binding studies of 4-(methylnitros-amino)-1-(3-pyridyl)-1-butanone (NNK) and its analogs with deuterium substitution at the positions α to the nitrosamino group ([4,4-D2]NNK and [CD3NNK) were carried out in A/J mice in order to assess the potential importance of DNA methylation or pyridyloxobutylation in lung tumor induction. The tumorigenic activities of the major NNK metabolite, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) and its analog with deuterium at the carbinol carbon ([1-D]NNAL) were also determined. Groups of A/J mice were given single i.p. injections of either 10 or 5 μmol of NNK, [4,4-D2]NNK, [CD3]NNK, NNAL and [1-D]NNAL, and were killed 16 weeks later. Lung tumor multiplicities were as follows in mice treated with 10 μmol: NNK, 7.3 ± 3.5; [4,4-D2]NNK, 1.4 ± 1.6; [CD3]NNK, 11.7 ± 5.4; NNAL, 3.2 ± 2.0; [1-D]NNAL, 3.2 ± 2.0. Similar relative tumorigenic activities were observed in mice treated with 5 μmol of these compounds. These results demonstrated that [4,4-D2]NNK was less tumorigenic than NNK and [CD3]NNK was more tumorigenic than NNK. NNAL was less tumorigenic than NNK; substitution of deuterium at the carbinol carbon did not affect its activity. Levels of O6-methylguanine (O6-mG) were measured in pulmonary DNA of A/J mice treated with 10 μmol of NNK, [4,4-D2]NNK or [CD3]NNK, and killed 2 or 24 h later. O6-mG levels were lower in mice treated with [4,4-D2]NNK than in those treated with NNK; no difference in O6-mG levels was observed between those treated with NNK and [CD3]NNK. The results of this study support the hypothesis that O6-mG formation in pulmonary DNA is the key step in lung tumor induction by NNK in A/J mice.