OXIDATIVE damage to DNA, reflected in the formation of 8-oxo- 7-hydrodeoxyguanosine (8-oxodG)^1,2, may be important in mutagenesis, carcinogenesis and the ageing process^3,4. Kuchino et al. studied DNA synthesis on oligodeoxynucleotide templates containing 8-oxodG, concluding that the modified base lacked base pairing specificity and directed misreading of pyrimidine residues neighbouring the lesion⁵. Here we report different results, using an approach in which the several products of a DNA polymerase reaction can be measured. In contrast to the earlier report⁵, we find that dCMP and dAMP are incorporated selectively opposite 8-oxodG with transient inhibition of chain extension occurring 3' to the modified base. The potentially mutagenic insertion of dAMP is targeted exclusively to the site of the lesion. The ratio of dCMP to dAMP incorporated varies, depending on the DNA polymerase involved. Chain extension from the dA · 8-oxodG pair was efficiently catalysed by all polymerases tested.