Neurogenesis is promoted by basic helix-loop-helix (bHLH) transcription factors Mash1, Math1 and/or NeuroD but suppressed by another set, Hes1 and Hes5. It is still unknown what kinds of extracellular signals are involved in their regulation. Therefore, the effects of neurotrophins on the expression of bHLH factors and neuronal differentiation were investigated by the use of cultured mouse neural stem cells. Each neurotrophin increased Mash1 and Math1 mRNAs of the stem cells growing in the presence of fibroblast growth factor-2 (FGF-2), but did not alter Hes1, Hes5 or NeuroD mRNA levels. Simultaneously, most of the cells expressed nestin, but not microtubule-associated protein 2 (MAP2), and still remained undifferentiated. FGF-2 removal from the medium reduced the levels of Hes1 and Hes5 mRNAs and increased those of Mash1, Math1, and NeuroD mRNAs, resulting in substantial neuronal differentiation. However, when the cells were pretreated with a neurotrophin, brain-derived neurotrophic factor, FGF-2 removal enhanced earlier NeuroD expression and generated many more MAP2-positive cells. The high level of Mash1 and Math1 that had been elevated at FGF-2 withdrawal accelerated NeuroD expression in cooperation with the reduced Hes1 and Hes5 expression. Our present results suggest that neurotrophins stimulate neuronal differentiation via altering the balance of expression of various bHLH transcription factors.