The present study was undertaken to determine the effects of neurotrophin-3 (NT3) and spontaneous bioelectric activity (SBA) on dendritic elongation and branching in long-term isolated organotypic explants of rat neocortex. Viral vector-directed expression of NT3 was used as an effective means to ensure a continuous, local production of the neurotrophic factor. Quantitative light microscopic measurement of dendritic branching patterns was carried out on Golgi-stained materials. Explants were exposed to an adenoviral vector encoding the genetic sequence for neurotrophin-3 (Ad-NT3), or to exogenous additions of the neuropeptide NT3. In order to test for activity-dependent growth effects under control and experimental conditions, explants were exposed to glutamatergic blockade using a cocktail of APV and DNQX. Both Ad-NT3 and NT3 peptide potently promoted apical and basal dendritic growth (elongation and branching) in pyramidal neurons. This growth was observed to be significant in layers II–IV and V. These growth effects were also not activity dependent, inasmuch as they were elicited from explants in which spontaneous bioelectric activity had been suppressed. Non-pyramidal neurons, throughout the neocortical slice, showed no significant dendritic responses to the prolonged presence of NT3. These findings show that pyramidal dendritic growth in long-term neocortical explants responds to at least one neurotrophic growth factor, NT3, and is independent of intrinsic bioelectric activity. The use of viral vectors in delivering a continuous high level of neurotrophic factor within developing neural tissues demonstrates its potential application to in vivo tissues during development, or in the stimulation of neuritogenesis and neuroregeneration following injuries.