Aliphatic N-nitrosomethylalkylamines exhibit a remarkable organ specificity in rats, the principal targets for tumour induction being liver, oesophagus, urinary bladder and lung. We have determined the extent of DNA methylation in these tissues following a single oral dose (0.1 mmol/kg; 6 h survival) of each of 12 homologues, ranging from N-nitrosodimethylamine (C1) to N-nitrosomethyldodecylamine (C12). Methylpurines (7- and O6-methylguanine) were determined by cation exchange HPLC with fluorescence detection. Highest levels of hepatic DNA methylation were found with N-nitrosodimethylamine (C1) and N-nitrosomethylethylamine (C2), the most potent hepatocarcinogens in this series. Concentrations of methylpurines in liver DNA decreased with increasing chain length for C1-C5. Administration of the higher homologues (C6-C12) caused levels of DNA methylation which by themselves were considered too low to account for their hepatocarcinogenicity. In rat oesophagus, DNA methylation closely paralleled carcinogenicity, the butyl and pentyl derivatives (C4, C5) being most effective. In rat lung, the extent of DNA methylation was generally lower and there was no apparent correlation with carcinogenicity. Methylation of kidney DNA also decreased with increasing chain length and was only detectable for C1-C5. In urinary bladder DNA, methylpurines were below or close to the limit of detection. It is concluded that the initiation of malignant transformation by DNA methylation alone (through hydroxylation at the methylene alpha-carbon) could be operative for C1 in kidney and lung, for C1 and C2 in liver, and C3-C5 in oesophagus. For the higher homologues, the extent of DNA methylation seems insufficient to explain the complex pattern of tissue specificity, suggesting that DNA modification other than, or in addition to, methylation may be responsible.