4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a major nitrosamine formed in tobacco smoke, induces a high incidence of lung, liver, and nasal cavity tumors in rats. Since alpha-hydroxylation of NNK by target tissues can lead to the generation of a methylating agent, the formation and removal of 7-methylguanine and the promutagenic lesions O6-methylguanine (O6mGua) and O4-methyldeoxythymidine were determined over 12 days of NNK administration to rats (100 mg/kg/day). DNA alkylation was greatest in the nasal mucosa, followed by liver and lung after 1 dose of NNK. No DNA adducts were detected in kidney and brain under these conditions. The concentration of O6mGua increased steadily in lung throughout the treatment regimen, while O6-methylguanine-DNA methyltransferase decreased to less than 5% of control. The concentration of O4-methyldeoxythymidine in lung DNA reached a steady state after 4 days of carcinogen treatment. After NNK treatment was discontinued, O6mGua persisted, while O4-methyldeoxythymidine was removed rapidly in the lung, suggesting that different repair pathways exist for the removal of these adducts in vivo. In hepatocytes, nonparenchymal cells, and nasal mucosa, O6mGua concentrations were maximal after 1-2 days and declined by 50-80% during the remaining 10 days of treatment. The decrease in O6mGua levels in nasal mucosa paralleled a decline in O6-methylguanine-DNA methyltransferase activity and was associated with marked cytotoxicity to Bowman's glands, portions of the lateral nasal gland, and the olfactory and respiratory mucosa during carcinogen treatment. In contrast, the decline in O6mGua in hepatocytes was attributed to the induction of O6-methylguanine-DNA methyltransferase activity, since an 18-fold reduction in the ratio of O6mGua:7-methylguanine was observed over the 12 days of treatment. These studies have demonstrated a marked accumulation of promutagenic DNA adducts in target tissues during repeated exposure to NNK.