The present study sought to determine the distribution of the cortical areas giving rise to the corticosubthalamic nucleus projections, using extracellular stimulating and recording techniques in rats with and without chronic lesions. In acute rats, cortical stimulation induced a powerful excitation in 87% of the subthalamic nucleus cells recorded. This response was obtained from stimulation over a large extent of the cortex since nearly all the ipsilateral cortex and the rostral two-thirds of the contralateral side was found to influence the activity of the subthalamic nucleus neurones. An excitatory response quite similar to that induced by cortical stimulation was recorded in the subthalamic nucleus after striatal or internal capsule stimulations. Therefore in order to eliminate the possibility of recording a polysynaptic excitation, similar experiments were performed in rats bearing various chronic lesions. With either ipsilateral or contralateral cortical stimulations, there was no major consequence of these lesions on the type or characteristics of the response recorded or on the percentage of responding cells. The cortical origin of the excitation of the subthalamic neurones was further supported by the results of experiments performed in chronically decorticated rats. It is concluded that (1) the subthalamic nucleus receives an excitatory cortical input, (2) this control comes from ipsilateral and contralateral cortical areas and (3) it only involves direct corticosubthalamic nucleus fibres. The subthalamic nucleus is, together with the striatum, the only basal ganglia nucleus known to receive afferents from extensive regions of the cortex. By its two main afferents (cortex and external segment of the pallidum), the subthalamic nucleus is in a position to compare direct cortical informations with cortical informations processed at the striatopallidal complex level.