There are two populations of neurons which are continually renewed in the adult, the dentate gyrus granule neurons and the olfactory bulb granule and periglomerular neurons. In the dentate gyrus, a secondary proliferative zone termed the subgranular zone is established along the interface between the dentate gyrus and the hilus where granule cells are born throughout life. Olfactory bulb neurons are generated in the anterior subventricular zone of the lateral ventricle and migrate via the rostral migratory stream to the olfactory bulb. We examined animals lacking brain-derived neurotrophic factor (BDNF) in order to establish whether this neurotrophin could be involved in the generation and/or survival of these neurons in vivo. We find that cells in nestin-positive regions of both the subgranular layer of the dentate gyrus and the subventricular zone of the olfactory bulb undergo apoptosis starting 2 weeks after birth in the absence of BDNF. However, increased apoptosis was not limited to precursors, as apoptotic cells were also found in the granule cell layer of the dentate gyrus and in the granule and periglomerular layers of the olfactory bulb. The excessive cell death was limited to these populations of neurons as no excessive cell death was detected in other forebrain areas. We conclude that BDNF is essential for the survival of neurons specifically in populations which are continuously being regenerated in the brain.