DURING normal development of the vertebrate nervous system, large numbers of neurons in the central and peripheral nervous system undergo naturally occurring cell death(1). For example, about half of all spinal motor neurons die over a period of a few days in developing avian, rat and mouse embryos(1). Previous studies have shown that extracts from muscle and brain, secreted factors from glia, as well as several growth factors and neurotrophic agents, including muscle-derived factors, can promote the survival of developing motor neurons in vitro and in vivo(2-15). But because neurotrophins and other known trophic agents administered alone or in combination are insufficient to rescue all developing motor neurons from cell deaths(8), other neurotrophic molecules are probably essential for the survival and differentiation of motor neurons. Here we report that glial-cell-line-derived neurotrophic factor (GDNF), a potent neurotrophic factor that enhances survival of mammalian midbrain dopaminergic neurons(16,17), rescues developing avian motor neurons from natural programmed cell death in vivo and promotes the survival of enriched populations of cultured motor neurons. Furthermore, treatment with this agent in vivo also prevents the induced death and atrophy of both avian and mouse spinal motor neurons following peripheral axotomy.