The spectrum of potential biological roles for neurotrophic factors in development and maturation of the nervous system continues to widen. Careful analysis of the phenotypes of knock-out mice has been used to test directly the 'neurotrophic hypothesis', and the role of members of the transforming growth factor beta superfamily--in particular, glial cell line derived neurotrophic factor--in regulating neuronal survival has become apparent. The effects of neurotrophin-3 on early neuronal differentiation and maturation have proved to be both multiple and complex. Neurotrophic factors are also emerging as potential regulators of synapse stabilization and function.