Using a quantitative reverse transcription-polymerase chain reaction (RT-PCR), we studied the regulation of trkA mRNA expression in serum-free, cultured basal forebrain neurons from 17-day fetal rats. Besides increasing choline acetyltransferase (ChAT) activities, nerve growth factor (NGF) strikingly induced trkA gene expression in a time- and NGF concentration-dependent manner. Therefore, NGF might play a critical role in trkA gene expression during the development of basal forebrain cholinergic neurons. Furthermore, to investigate whether this up-regulation is connected with the trophic effects on basal forebrain cholinergic neurons, we examined the effects of some other neurotrophic agents (BDNF, NT-3, bFGF, CNTF, and 40 mM KCI) upon ChAT activity and trkA gene expression. Some neurotrophic factors increased ChAT activities to the same degree as NGF, whereas they did not stimulate trkA mRNA expression so potently. NT-3 plus K252b promotes the tyrosine phosphorylation of TrkA in PC12 cells and increases ChAT activity in cultured basal forebrain cholinergic neurons like NGF (Knusel et al., J Neurochem 59: 715-722, 1992). We found that NT-3 plus K252b upregulated the level of trkA mRNA. These results suggested that the expression of trkA mRNA is regulated directly by its specific ligand NGF, rather than neurotrophic effects upon basal forebrain cholinergic neurons and that the up-regulation is connected to a molecular event initiated by the binding of NGF to the TrkA receptor.