Autoradiographic studies reveal densities of binding to somatostatin, neurotensin, μ-opiate, and benzodiazepine receptors in substantia nigra specimens from neurologically normal human brains. Binding to nigral angiotensin converting enzyme is also dense, whereas more modest densities of κ-opiate, dopamine, and serotonin receptors are noted. In nigral specimens taken from patients with idiopathic Parkinson's disease, substantial reductions in somatostatin, neurotensin, μ-opiate and κ-opiate receptors contrast with more modest reductions in dopamine and benzodiazepine I receptor subtypes. Angiotensin converting enzyme, serotonin, and benzodiazepine II binding are virtually unaltered. These results underscore the likelihood of strong peptidergic influences on normal and pathologically altered human nigrostriatal circuitry.