1. In view of previously demonstrated modulatory effects of PLG on the sensitivity of central dopamine receptors, we developed a radioligand binding assay to identify specific binding sites of PLG in rat and normal human brain. 2. 3H-PLG binds specifically to rat striatum exhibiting high affinity (KD = 4.69 +/- 0.50 nM) saturability (Bmax = 9.20 +/- 0.30 fmoles/mg protein) and reversibility; the highest density of specific PLG binding sites occurring in the striatum, followed by the hypothalamus and cerebral cortex. 3. Saturable, high-affinity binding sites of PLG were identified in human striatum. The substantia nigra was enriched with the highest level of specific PLG binding sites. 4. Dopamine receptors were identified in human lymphocytes. 5. The results are compatible with the hypothesis that differential modulation of CNS dopamine receptors by PLG is functionally associated with interacting with specific PLG binding sites in the rat and human brain, and pose implications for Parkinson's disease and tardive dyskinesia.