Nerve growth factor (NGF) appears to act as a neurotrophic factor for basal forebrain and caudate-putamen cholinergic neurons. The mechanism by which NGF transduces its signal in these neurons is yet to be defined. Recent data indicate that the product of the trk gene, p140trk is a critical component of the NGF receptor. Herein, we show that p140trk mRNA is highly restricted in its distribution in the adult rat forebrain, that it is present in cholinergic neurons, and that most if not all cholinergic neurons contain p140trk mRNA. Furthermore, induction of trk expression by NGF suggests that neurotrophin-mediated up-regulation of their receptor tyrosine kinases is an important feature of their actions and that neurotrophins may regulate the activity of responsive neurons through increasing the level of their receptors.