Calf thymus DNA and the synthetic polynucleotides of defined sequence, poly(dG).poly(dC) and poly(dG-dC).poly(dG-dC) were reacted with N-methyl-N-nitrosourea (MNU) and the amounts of 7-methylguanine (7-MG) and O6-methylguanine (O6-MG) determined. It was found that the O6-MG/7-MG ratio for DNA was 0.13, for poly(dG).poly(dC) was 0.10, and for poly(dG-dC).poly(dG-dC) was 0.21. From these data, it is evident that the neighboring bases affect the alkylation pattern of guanine by MNU, a finding that can have significant relevance in defining 'hot spots' of alkylation in the genome which may be important in the processes of mutagenesis and carcinogenesis.