Neurotrophic effects resulting from the insulin/insulin receptor system have been recognized as important in determining the etiological basis of neurodegenerative disorders. In Parkinson's disease, selective neuronal loss in the substantia nigra is accompanied by decreased immunoreactivity of the insulin receptor as determined using immunohistochemical studies. We performed semiquantitative mRNA analysis by reverse transcription-polymerase chain reaction (RT-PCR) using specific primers for human insulin receptor exon 22, which encodes a region of the beta subunit of the receptor serving as a tyrosine kinase domain. The relative levels of mRNA in the substantia nigra from Parkinson's brain tissues showed a marked depression compared with those of normal controls. Further investigations are needed to decide whether this is a primary, disease-specific alteration of gene expression or merely a secondary process.