The contribution of the dopamine‐synthetic capacity of nigral neuronal subregions to their vulnerability to degeneration in idiopathic Parkinson's disease (IPD) was explored using semiquantitative in situhybridization to study expression of mRNA encoding the rate‐limiting dopamine synthetic enzyme, tyrosine hydroxylase (TH). Expression of mRNA, the structural protein, β‐tubulin, and the glycolytic enzyme, fructose‐1,6, biphosphate aldolase (aldolase C) was studied in parallel in individual neurons of the substantia nigra pars compacta (SNc) in matched groups of IPD and control subjects. TH mRNA expression was found to be heterogeneously expressed in nigral neurons in control and IPD subjects. There was no significant difference in mean values for TH mRNA expression between control and IPD cases and none between nigral subregions, either in control subjects or in established IPD subjects in this study, but there was evidence for a selective upregulation of TH mRNA expression in non‐melanized neurons in IPD. There was no relationship between TH mRNA expression disease duration or L‐dopa dosage in the IPD group. Mean TH mRNA values for two additional 40‐year‐old control subjects fell within the range of values of the aged‐control group. Aldolase C and β‐tubulin expression did not differ between control and IPD groups or between nigral subregions.