Responses of the subthalamic nucleus (STH) neurons to the stimulation of the sensorimotor cortex (Cx) were recorded in intact rats and in those which received lesions in the pallidum, the neostriatum, the brainstem, or the corpus callosum. Most of the STH units (78%) exhibited two excitatory peaks which were interrupted by a brief period of inhibition. Some of units which were located in the peripheral part of the STH tended to lack the brief inhibitory component and exhibited a long period of excitation. These excitations were followed by a long-lasting inhibitory period. Intracellular recording indicated that these responses were EPSPs interrupted by a short IPSP and a long period of disfacilitation of Cx inputs. A quinolinic acid lesion of the neostriatum and a knife cut of the brainstem failed to alter these responses, while an ibotenic acid lesion of the globus pallidus abolished the short inhibition seen in the midst on the excitation. Stimulation of contralateral Cx also evoked excitatory responses in the STH. The responses were completely eliminated by a parasagittal knife cut of the rostral part of the corpus callosum.