Activation of group III metabotropic glutamate (mGlu) receptors has been recently highlighted as a potential approach in the treatment of Parkinson's disease (PD). This study evaluates the antiparkinsonian action of systemic administration of the broad-spectrum agonist of group III mGlu receptors, 1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I), and investigates its site of action within the basal ganglia circuitry. Acute injection of ACPT-I reverses haloperidol-induced catalepsy, an index of akinesia in rodents. In a rat model of early PD based on partial bilateral nigrostriatal lesions, chronic (2weeks) administration of ACPT-I is required to efficiently alleviate the akinetic deficit evidenced in a reaction time task. This treatment counteracts the post-lesional increases in the gene expression of cytochrome oxidase subunit I, a metabolic marker of neuronal activity, in the overall subthalamic nucleus and in the lateral motor part of the substantia nigra pars reticulata (SNr) but has no effect in the globus pallidus. Paradoxically, ACPT-I administration in sham animals impairs performance and induces overexpression of cytochrome oxidase subunit I mRNA in the lateral SNr, and has no effect in the subthalamic nucleus or globus pallidus. Altogether, our results provide new evidence for the antiparkinsonian efficiency of group III mGlu receptor agonism, point to the regulation of the overactive subthalamo-nigral connection as a main site of action in an early stage of PD and underline the complex interplay between these receptors and the dopaminergic system to regulate basal ganglia function in control and PD conditions.