The role of neurotrophin-3 (NT-3) in early development of the dorsal root ganglion was investigated. Excessive cell death in the dorsal root ganglion of mice that carry a deleted NT-3 gene (NT-3-/- mice) preceded the period of programmed cell death, detected by the TUNEL method, and caused a reduction in the number of proliferating precursors without altering the proportion of proliferating cells to total number of neurons. Furthermore, the majority of proliferating cells detected by bromodeoxyuridine incorporation also stained with the TUNEL method. NT-3 mRNA was expressed locally in the embryonic, but not the postnatal dorsal root ganglion. Most cultured early embryonic NT-3-/- neurons died in the absence of exogenous NT-3 as did the wild-type neurons when cultured with NT-3 neutralizing antibodies, suggesting that NT-3 acts locally to prevent the death of proliferating sensory precursor cells during neurogenesis. Thus, NT-3 may inflict constraints on the number of proliferating precursor cells and thereby affect the number of neurons generated during development of the peripheral nervous system.