Neurotrophin-3 (NT-3) is a member of the neurotrophin gene family and is highly expressed in the developing rat cerebellum. Here we show that brain-derived neurotrophic factor (BDNF) increased by approximately 10-fold the NT-3 mRNA levels in cultured cerebellar granule neurons isolated from postnatal rats, whereas nerve growth factor (NGF) and NT-3 itself had no effect. The effect of BDNF was additive to that of triiodothyronine (T3), which also increased NT-3 mRNA in these neurons. The drug K252a inhibited the BDNF-mediated stimulation of NT-3 expression, suggesting an involvement of trkB receptors. Nuclear run-on experiments showed that BDNF enhanced NT-3 transcription, whereas the stability of NT-3 mRNA remained unchanged. The data presented are the first demonstration that one neurotrophin regulates the expression of another and provide evidence that NT-3 production in granule neurons is regulated by both BDNF and T3.