The comparative mutagenicity of 8-oxo-7,8-dihydro-2′-deoxyadenosine (8-oxodA) and 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxodG) was explored using simian kidney (COS-7) cells. Oligodeoxynucleotides [5′-TCCTCCT- G₁X₂ CCTCTC or 5′-TCCTCCT X₁G₂ CCTCTC (X = dA, dG, 8-oxodA or 8-oxodG)] containing 8-oxodA or 8-oxodG positioned within codon 60 or 61 of the non-coding strand of human c-Ha- ras1 gene were inserted into a single-stranded phagemid shuttle vector. The vector was replicated in COS-7 cells and the progeny plasmids were used to transform Escherichia coli DH10B. The transformants were analyzed by oligodeoxynucleotide hybridization and DNA sequence analysis to establish the mutation frequency and specificity. When 8-oxodA was positioned at X ₁ , targeted A ^oxo →C transversions were detected; the mutation frequency was 1.2%. When 8-oxodA was positioned at X ₂ , one targeted mutant among 416 colonies screened (an A ^oxo →G transition) was detected. Thus, the mutation frequency and spectrum of 8-oxodA depend on the sequence context of the lesion. The mutation frequency of 8-oxodG at X ₁ and X ₂ was 5.2 and 6.8%, respectively. G ^oxo →T transversions dominated the spectrum, accompanied by small numbers of G ^oxo →A transitions and G ^oxo →C transversions. We conclude that 8-oxodA has mutagenic potential in mammalian cells, generating A→C transversions. However, when tested under similar conditions, the mutation frequency of 8-oxodA is at least four times lower than that of 8-oxodG.