In situ hybridization for TrkA mRNA was combined with quantitative optical densitometry to evaluate whether the expression of this gene is altered within cholinergic basal forebrain neurons (CBF) in Alzheimer's disease (AD). TrkA mRNA within individual nucleus basalis neurons was significantly reduced (66%) in AD cases relative to aged controls. Reverse transcription polymerase chain reaction quantitative analyses confirmed that TrkA mRNA levels decreased markedly in AD. In contrast, expression of the gene coding for for the low affinity p75NTR was not significantly altered in AD relative to aged controls. These data indicate that there is a selective defect in trkA gene expression in AD, supporting the hypothesis that the degeneration of CBF neurons seen in this disease results from impaired nerve growth factor trophic support.