Chronic L-dopa treatment of Parkinson's disease (PD) often leads to debilitating involuntary movements, termed L-dopa-induced dyskinesia (LID), mediated by dopamine (DA) receptors. RGS9 -2 is a GTPase accelerating protein that inhibits DA D2 receptor-activated G proteins. Herein, we assess the functional role of RGS9 -2 on LID. In monkeys, Western blot analysis of striatal extracts shows that RGS9 -2 levels are not altered by MPTP-induced DA denervation and/or chronic L-dopa administration. In MPTP monkeys with LID, striatal RGS9 -2 overexpression -achieved by viral vector injection into the striatum -diminishes the involuntary movement intensity without lessening the anti-parkinsonian effects of the D1/D2 receptor agonist L-dopa. In contrasts, in these animals, striatal RGS9 -2 overexpression diminishes both the involuntary movement intensity and the anti-parkinsonian effects of the D2/D3 receptor agonist ropinirole. In unilaterally 6-OHDA-lesioned rats with LID, we show that the time course of viral vector-mediated striatal RGS9 -2 overexpression parallels the time course of improvement of L-dopa-induced involuntary movements. We also find that unilateral 6-OHDA-lesioned RGS9 Ϫ/Ϫ mice are more susceptible to L-dopa-induced involuntary movements than unilateral 6-OHDA-lesioned RGS9 ϩ/ϩ mice, albeit the rotational behavior -taken as an index of the anti-parkinsonian response -is similar between the two groups of mice. Together, these findings suggest that RGS9 -2 plays a pivotal role in LID pathophysiology. However, the findings also suggest that increasing RGS9 -2 expression and/or function in PD patients may only be a suitable therapeutic strategy to control involuntary movements induced by nonselective DA agonist such as L-dopa.