Abstract Gene therapy in the nervous system offers an attractive strategy for the administration of therapeutic factors in a potentially region-specific, sustained, and well-tolerated manner. We tested the trophic effect of a recombinant adenovirus encoding nerve growth factor (AdNGF)in vivoon basal forebrain cholinergic neurons of aged rats, a neuronal population affected during normal and pathological aging. Three weeks after unilateral injection of the recombinant adenovirus into the nucleus basalis magnocellularis, a significant increase in the somal areas of cholinergic neurons ipsilateral to the injection was observed. No increase was detected in animals receiving a recombinant adenovirus carrying theEscherichia coli Lac Zreporter gene. Injected animals did not lose weight, an adverse effect usually described after intracerebroventricular infusion of NGF, and no tissue loss or massive local inflammatory response was observed around injection sites. Thus, a single intracerebral injection of AdNGF produces trophic effects similar to those resulting from chronic intracerebroventricular high levels of NGF. These findings indicate that recombinant adenoviruses encoding growth factors are potentially powerful tools for improving neuronal deficits associated with degenerative processes.