The capacity of Chinese hamster ovary cells for excising alkylated bases from their DNA was investigated. Cells were treated with N-methyl-N-nitrosourea or N-ethyl-N-nitrosourea, and the amounts of 3-alkyladenine, 7-alkylguanine and O6-alkylguanine in DNA were compared immediately after the treatment and after a 20-h incubation. Cells were able to excise 3-alkyladenine and 7-alkylguanine, but there was no loss of O6-alkylguanine, after correction for dilution, due to DNA synthesis during post-treatment incubation. The inability of Chinese hamster ovary cells to excise O6-alkylguanine may be the reason that Chinese hamster ovary cells fail to show growing point mutagenesis after treatment with N-ethyl-N-nitrosourea.