The presence of acid and alkaline DNases in nuclei of chick brain cells has been demonstrated. The activities of these two DNases along with those of DNA-polymerases were assessed in chicken tissues with known varied cell proliferative capacities (eg., spleen, kidney and brain) at different ages. The results indicate that the acid and alkaline DNases are probably the 'house keeping' enzymes with a constitutive role in DNA repair process, the former with a DNA-repair process that is linked to cell proliferation (DNA synthesis) while the latter with the basal DNA-repair operations that must go on at all times without any regard to the cell division process. Chicken brain, unlike that of rat, possesses significant levels of aphidicolin sensitive DNA-polymerase(s) that are considered to be more replication oriented enzymes suggesting that the replication potential of adult and old avian brain cells may be different from that of a mammalian brain.