Prior work has shown that intrastriatal injection of the metabotropic glutamate receptor agonist 1S, 3R-ACPD results in pronounced contralateral rotation, and the basis for this effect is thought to be increased activity of dopaminergic nigrostriatal neurons. We tested this hypothesis by determining the expression of Fos-like immunoreactivity after intrastriatal injection of 1S, 3R-ACPD. Intense Fos-like immunoreactivity was noted in the globus pallidus, entopeduncular nucleus, subthalamic nucleus and substantia nigra pars reticula. Ablation of the subthalamic nucleus 10 days prior to intrastriatal injection of 1S, 3R-ACPD abolished rotational behaviour but not Fos-like immunoreactivity in the globus pallidus, entopeduncular nucleus and substantia nigra. Intrasubthalamic injection of 1S, 3R-ACPD produced marked contralateral rotation and a pattern of Fos-like immunoreactivity similar to that seen after intrastriatal 1S, 3R-ACPD injection. These results suggest that stimulation of striatal metabotropic glutamate receptors inhibits striatal projection neuron activity, while stimulation of subthalamic metabotropic glutamate receptors increases subthalamic nucleus activity. Increased subthalamic nucleus activity is necessary and sufficient for the expression of rotational behavior. These results also suggest that metabotropic glutamate receptor antagonists may be useful in the treatment of Parkinson's disease.