The binding properties of mu and delta opioid receptors were investigated in several areas of human brain by using [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol and [3H]Tyr-D-Thr-Gly-Phe-Leu-Thr as respective selective ligands, while the totality of opioid receptors was measured by using [3H]etorphine as a non-selective agonist. Receptor densities were highest in cerebral cortex, amygdala and striatum, and lowest in the substantia nigra (pars compacta). In the different brain areas of patients with Parkinson's disease, the density and the proportion of the various opioid receptors were not significantly different from control subjects.