The molecular dosimetry of O6-methylguanine (O6-meG) in DNA from lung and nasal mucosa was determined during administration of 4-(N-nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK) to Fischer 344 rats. O6-MeG accumulated in lung during 12 days of treatment with doses of NNK ranging from 0.1 to 100 mg/kg per day. The dose-response to NNK was nonlinear; the ratio of O6-meG to dose, an index of alkylation efficiency, increased dramatically as the dose of carcinogen decreased. These data suggest that high and low Km pathways may exist for activation of NNK to a methylating agent. Clara cells, when compared to Type-II cells, macrophages and alveolar small cells, were found to possess the greatest concentration of O6-meG. Moreover, as the dose of NNK was decreased, a marked increase in the alkylation efficiency of Clara cells was observed. Thus, the presence of a high-affinity, low-Km pathway in Clara cells for activation of NNK may be a significant factor in the carcinogenicity of this tobacco-specific carcinogen. The dose-response for O6-meG differed considerably between respiratory and olfactory mucosa. The dose-response to NNK was nonlinear in respiratory mucosa and linear in the olfactory mucosa, and the concentration of O6-meG was five times greater in respiratory than in olfactory mucosa after treatment with 1 mg/kg NNK. As the dose of NNK was increased, alkylation in the two regions of the nose became similar. Histological examination of the nasal passages following treatment with NNK indicated that the olfactory region was more sensitive than the respiratory region to toxicity induced by NNK.(ABSTRACT TRUNCATED AT 250 WORDS)