Nuclear poly(ADP-ribose) polymerase levels as well as the DNA strand break levels of whole-brain neuronal and astroglial cells were investigated . Three-and 30month-old rats were used . Low-molecular-weight neurofilaments and glutamine synthetase served as neuronal and astroglial markers, respectively . A large increase in the poly(ADP-ribose] polymerase activity was observed in the neurons (threefold) and astrocytes (3 .7-fold) derived from 30-month-old rats . Similarly, the amount of poly(ADP-ribose) polymerase, evaluated per milligram of DNA, in-creased~3 .5-fold in neurons and 3 .9-fold in astrocytes prepared from 30-month-old rats . Whether the increase in the poly(ADP-ribose) polymerase activity was due to an enhanced rate of DNA strand break was investigated by determining the rate of DNA unwinding . A significant increase in DNA unwinding rate was detected in the neurons {2 .7-fold}, although a lower increase was observed in the astroglia (1 .3-fold) of aged animals .