Alkylation of liver DNA was studied following administration to Sprague-Dawley rats of doses of dimethylnitrosamine (DMN) varying from 0.25 to 20 mg/kg body weight. Measurements were made of the amounts of O6-methylguanine and 7-methylguanine present in liver DNA at 4 and 24 hours after treatment with the carcinogen. There was a linear relationship between 7-methylguanine levels and dose of the nitrosamine at both of these times. In contrast, the corresponding levels of O6-methylguanine were not directly proportional to dosage but were less than expected, particularly at low doses below 2.5 mg/kg. This discrepancy was significant at 4 hours, but was even more marked at 24 hours. Only doses above 4 mg/kg at the 4-hour time point gave rise to a 0.11 ratio of alkylation of guanine at the O6-position to that at the 7-position. This ratio was that expected for the initial interaction of the alkylating species derived from DMN with DNA. Evidence was obtained to support the hypothesis that these results were due to an enzymatic removal of O6-methylguanine from liver DNA, which occurred much more efficiently at lower initial levels of alkylation. Repeated daily injections of DMN up to 11 days alos gave rise to O6-methylguanine levels that were not proportional to dosage but were relatively greater at higher dose levels. The significance of these findings in the induction of liver cancer by feeding or repeated injection of DMN was explored.