The development and maintenance of the vertebrate nervous system depends upon neuronal survival proteins known as neurotrophic factors. Nerve growth factor (NGF) remains the best characterized neurotrophic molecule. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) are two recently cloned neurotrophic factors that are homologous to NGF. Here we describe the molecular cloning of the human and rat genes encoding BDNF, as well as the isolation of the human NT-3 gene. On the basis of comparison of our genomic and cDNA clones with those of previously isolated BDNF and NT-3 genes and cDNAs, we make inferences about the structures of processed transcripts derived from the neurotrophin genes and the protein precursors they encode. We demonstrate that the mature form of BDNF is identical in all mammals examined, and that the same is true of the mature form of NT-3. Furthermore, the respective tissue-distributions and neuronal specificities of NT-3 and BDNF are also conserved among mammals. Finally, we localize the gene encoding human BDNF (gene symbol designated BDNF) to chromosome 11, band p13, and the gene encoding human NT-3 (gene symbol designated NTF3) to chromosome 12, band p13.