The synaptic effect of the projection from an intralaminar thalamic structure, the parafascicular nucleus, to the subthalamic nucleus was investigated through extracellular recordings of subthalamic unit activities. Electrical stimulation of the parafascicular nucleus caused a complex response with two successive excitatory phases. The first excitation was assumed to be monosynaptically driven since it was not affected by pallidal lesion or transsection of the internal capsule. Pharmacological activation of the parafascicular neurons through microinjections of carbachol elicited a prolonged increase in the subthalamic discharge rate. These results suggest that the intralaminar parafascicular nucleus contributes to the activation of subthalamic neurons.