Mutations at the neurofibromatosis 1 (NF1) locus in humans and mice result in abnormal growth of neural crest-derived cells, including melanocytes and Schwann cells. We have exploited a targeted disruption of the NF1 gene in mice to examine the role of neurofibromin in the acquisition of neurotrophin dependence in embryonic neurons. We show that both neural crest- and placode-derived sensory neurons isolated from NF1(-/-) embryos develop, extend neurites, and survive in the absence of neurotrophins, whereas their wild-type counterparts die rapidly unless nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) is added to the culture medium. Moreover, NF1 (-/-) sympathetic neurons survive for extended periods and acquire mature morphology in the presence of NGF-blocking antibodies. Our results are consistent with a model wherein neurofibromin acts as a negative regulator of neurotrophin-mediated signaling for survival of embryonic peripheral neurons.