This study investigated the effects of exogenous nerve growth factor (NGF) on the survival and differentiation in primary culture of sensory neurons isolated from adult (6 months) and aged (2 years) mice. For neurons prepared from adult mice, a concentration effect was evident during a 2 week culture period: Neuronal counts in cultures supplemented with 25 and 50 ng/ml NGF did not differ significantly from those of control cultures without exogenous NGF or those with anti-NGF included in the culture medium, whereas cultures supplemented with either 100 or 200 ng/ml NGF contained higher numbers of neurons throughout the culture period. Cultures prepared from aged mice contained less neurons than those from adult mice, although those supplemented with 100 ndml NGF retained higher neuronal numbers than cultures from aged mice which did not receive exogenous NGF. Neuronal diameters were measured to investigate whether specific subpopulations of neurons were more dependent on NGF; the results indicate that neurons of a medium-larger diameter were more prevalent than cells with a smaller diameter following NGF administration. A shape index was calculated for each culture regimen; with longer culture periods a higher proportion of spindle-shaped neurons was observed.