In vivo studies of nerve tissue of senescent animals have raised the possibility that initiation and elongation of axonal processes in regeneration are somehow altered by aging. However, this has not been examined in dissociated cell culture because of technical difficulties. The present study was performed to compare the effects of nerve growth factor (NGF) and gangliosides mixture (Gg1) on neurite elongation of dorsal root ganglion (DRG) cells from senescent mice with those from young adult mice. DRG neurons were obtained from male mice of the 72-week-old (Group O) and of the 10-week-old (Group Y). Dissociated neuronal cells of each group were cultured for 4 days in chemically determined serum-free medium including various levels of 7S-NGF up to 160 ng/ml or of Gg1 up to 20 micrograms/ml. After immunohistochemical identification of neuronal cells, neurites with two kinds of lengths, maximum length (ML; length of the longest neurite belonging to the individual neuron) and total length (TL; summation of length of all neurites belonging to that) were quantified as elongated neurites. NGF showed concentration-dependent neurite elongation in ML and TL of Group O and Y. Gg1 increased TL at relatively low levels in both groups but no significant changes were detected in ML. On the other hand high levels of Gg1 did not show any enhancement in ML or in TL. These results indicate that the effects of NGF and Gg1 were not altered by aging, as far as elongation of neurite was concerned. In addition to biochemical deterioration of the neuron itself, decreased release of neurite-promoting factors such as NGF and Gg1 from target cells of the neuron and surrounding cell may account for the reduction of capacity for neurite regeneration in aged animals.