Substance P (SP), Met-enkephalin (Met-enk) and cholecystokinin-8-S (CCK-8-S) were measured by a combined HPLC/RIA method in the caudate nucleus and anterior putamen from controls and from Parkinson's disease (PD) patients. SP levels were reduced in caudate in PD, but unchanged in putamen. No differences in Met-enk content were found in parkinsonians compared to controls. However, a significant correlation between DA and Met-enk levels in caudate nucleus from PD was observed. The concentration of CCK-8-S was unaltered in caudate nucleus or putamen in PD. The decrease in caudate nucleus SP levels might be related to the decrease in nigral SP levels in PD, while the reduction in Met-enk levels appears to be a feature of a subgroup of parkinsonian patients.