Abstract The alkylation of a number of purified tRNA preparations by reaction with the carcinogens, N-methyl- N-nitrosourea and N-ethyl- N-nitrosourea was studied in order to investigate the role of nucleic acid structure on the distribution of alkylation products within the nucleotide sequence. The rate of alkylation was greatly increased by increasing the pH over the range 6 to 8 and the degree of alkylation (expressed as moles alkyl groups/mole tRNA) was directly proportional to the concentration of the nitrosamide added and independent of the amount of tRNA present. There was no significant difference in the degree of alkylation of any of the tRNA preparations tested. Reaction with N-ethyl- N-nitrosourea resulted in a degree of alkylation some 13 times less than that produced by reaction with a similar concentration of N-methyl- N-nitrosourea. The major product of the reaction was 7-alkylguanine amounting to about 80% of the total, but 3-methylcytosine, 6- O-methylguanine and 1-methyl-, 3-methyl-, and 7-methyladenine were also identified as products of the reaction of tRNA fMet with N-methyl- N-nitrosourea. The possibility that preferential alkylation of certain residues within the polynucleotide sequence was produced by reaction with the nitrosamides was examined by degradation of the alkylated tRNA with pancreatic ribonuclease and separation of the oligonucleotide fragments by chromatography on DEAE cellulose. When tRNA fMet which had been alkylated by reaction with N-methyl- N-nitrosourea or N-ethyl- N-nitrosourea was analysed in this way, the distribution of 7-alkylguanine was, within the limits of experimental error, in agreement with that expected for a random reaction of the alkylating agent with all of the guanosine residues throughout the molecule. A similar result was seen when tRNA Phe was examined. These results were obtained by alkylation under conditions where the native configuration of the tRNA was maintained and show that the tertiary structure of the nucleic acid does not impart any specificity to the reaction with the nitrosamide producing 7-alkylguanine but the possibility that such specificity does exist for the minor products of alkylation cannot be excluded.