Neurons do not divide during adult life and thus they provide a unique system to study the effects of age-accumulated damage to DNA in the absence of DNA replication. We have analyzed DNA polymerase activity in neurons isolated from young adult and very aged mice. The predominant catalytic activity is DNA polymerase-beta and it is present in similar amounts in neurons from young and old mice. This polymerase is highly error-prone in copying phi X174 DNA, the error frequency being about 1/7,000 and not significantly different when obtained from young and old animals. This high infidelity is considered with respect to DNA repair and the protein synthesis error catastrophe theory of aging.