Brain-derived neurotrophic factor (BDNF), a neurotrophin, enhances the survival and differentiation of several classes of neurons in vitro. To determine its essential functions, we have mutated the BDNF gene. Most homozygote mutants die within 2 days after birth, but a fraction live for 2-4 weeks. These develop symptoms of nervous system dysfunction, including ataxia. The BDNF mutant homozygotes have substantially reduced numbers of cranial and spinal sensory neurons. Although their central nervous systems show no gross structural abnormalities, expression of neuropeptide Y and calcium-binding proteins is altered in many neurons, suggesting they do not function normally. In contrast with mice lacking the BDNF receptor TrkB, motor neurons appear normal in the BDNF mutant.