The specific binding of [3H]neurotensin, [3H]substance P, [3H]D-Ala2-D-Leu5-enkephalin (delta receptors) and [3H]-Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (mu receptors) were studied in membrane preparations of caudate nucleus, putamen, globus pallidus and substantia nigra from patients with Parkinson's disease and from age-matched controls. The density of neurotensin receptors was decreased in globus pallidus (lateral and medial segments) in parkinsonian brain. Substance P receptors were reduced in the putamen (anterior and posterior) and in lateral globus pallidus in Parkinson's disease. There was a reduction in the density of opioid receptors in posterior putamen and in mu receptors in caudate nucleus and putamen (anterior and posterior). No differences in neuropeptide receptor binding were observed in substantia nigra from parkinsonian brains compared with control subjects. The reductions in neuropeptide receptor density were less marked than the decrease in caudate and putamen content of dopamine and its metabolites. This suggests that neuropeptide receptors are only partially localized to striatal dopamine terminals.