In monkeys rendered parkinsonian with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), expression of the dopamine D₃ receptor was decreased. However, levodopa-induced dyskinesia (LID), similar to the debilitating and pharmacoresistant involuntary movements elicited after long-term treatment with levodopa in patients with Parkinson disease (PD), was associated with overexpression of this receptor. Administration of a D₃ receptor–selective partial agonist strongly attenuated levodopa-induced dyskinesia, but left unaffected the therapeutic effect of levodopa. In contrast, attenuation of dyskinesia by D₃ receptor antagonists was accompanied by the reappearance of PD-like symptoms. These results indicated that the D₃ receptor participated in both dyskinesia and the therapeutic action of levodopa, and that partial agonists may normalize D₃ receptor function and correct side effects of levodopa therapy in patients with PD.