The developing cerebellum expresses genes which encode for both neurotrophins and their receptors. The present study was designed to determine at what stages during cerebellar granule cell neurogenesis neurotrophin family molecules may act. We report here that in purified, well-characterized granule cell cultures (Gao et al., 1991; 1992), none of the neurotrophins stimulated proliferation of granule cell precursors or rescued phenotypic defect of mutant weaver granule cell precursors in the initiation of neuronal differentiation. However, neurotrophin-4/5 (NT-4/5) and BDNF, but not neurotrophin-3 (NT-3) or NGF, promoted neurite extension and survival of differentiated cerebellar granule cells. Both of these effects were blocked by the specific inhibitor for Trk tyrosine kinases, K-252a. NT-4/5 and BDNF also enhanced neurite extension by weaver granule cells which were rescued by wild-type granule cells during differentiation. Moreover, TrkB immunohistochemistry performed on sections of the developing wildtype and weaver cerebella revealed that only differentiated granule cells, but not the precursor cells, make high levels of TrkB receptor. These findings together suggest that NT-4/5 and BDNF promote the maturation and maintenance of differentiated granule cells, effects which are downstream to the weaver gene. Since no additive effects were seen with the combination of NT-4/5 and BDNF, it seems likely that the two neurotrophins activate the same receptor trkB for signal transduction.