In the cerebellum, the mRNAs for neurotrophin-3 (NT-3) and its high-affinity tyrosine kinase receptor trkC are expressed by both the differentiated granule cells of the internal granule cell layer (IGL) and their precursors in the external germinal layer (EGL). We have investigated the effects of chronic application of exogenous NT-3 in vivo on cerebellar granule cell genesis and differentiation. NT-3 was applied to the posterior surface of the rat cerebellum from P6 onwards using Elvax implants. At P10 the EGL of cerebellar lobules VII and VIII was significantly reduced in thickness in NT-3 implanted rats when compared with controls. Immunocytochemical analysis of the EGL using antibodies to proliferating cell nuclear antigen (PCNA) revealed that the number of postmitotic, premigratory (PCNA-immunonegative) granule cell precursors was preferentially reduced in the NT-3 implanted rats. In situ DNA fragmentation labelling confirmed that this was not accompanied by increased cell death in the EGL. These results suggest that NT-3 promotes the differentiation of postmitotic, premigratory granule cell precursors, accelerating cell exit from the EGL.