We review what is known regarding the functional role of newly born cells, highlight the role of BDNF in this process, and describe preliminary findings from our lab implicating BDNF in the process of selecting of newly born cells for survival.;Inhibition of TrkB/C also caused a delay in the generation of neurons, but not astrocytes, and ultimately perturbed the postnatal localization of cortical neurons in vivo. Conversely, overexpression of BDNF in cortical precursors in vivo promoted proliferation and enhanced neurogenesis.;In both experimental paradigms we observed significantly more BrdU-labeled cells in the olfactory bulbs on the BDNF-infused side than in the olfactory bulb of PBS-infused animals.;Finally, BDNF prevents motor neuron death in the axotomized facial nucleus of the neonatal rat.;The generalization of the concept that neurotrophic proteins regulate neuronal survival during normal development critically depends on the demonstration that the survival of neurons in vivo can be increased by the administration of a neurotrophic protein different from NGF. We report here that this is the case with brain-derived neurotrophic factor, a protein of extremely low abundance purified from the central nervous system.;Analysis of motoneuron numbers indicated that both BDNF and GDNF provided protection to developing spinal cord motoneurons from ethanol toxicity, restoring motoneuron numbers to control levels.;These specific in vivo effects of BDNF and NT-4/5 on axonal regeneration from injured RGCs may be used to promote growth and expand the abnormally small terminal arbors observed when RGCs regrow into their CNS targets.