The benefits of stereotactic ventral posteromedial pallidotomy in the treatment of Parkinson's disease have been recently rediscovered. Optimal lesion location and lesion volume, however, have yet to be determined. Micro-electrode recording and microstimulation are carried out to determine an appropriate site for the placement of electrocoagulation lesions in the medial pallidum. The cellular activity of the globus pallidus is examined for characteristic firing patterns, mean firing rates, movement-evoked activity, and presence of tremor cells, laminae, and border zones. Microstimulation allows the identification of the adjacent optic tract by reports of visual sensation and of the internal capsule by sensorimotor responses. Lesions are centred at sites in the internal segment of the globus pallidus at least 3 mm from these structures, to avoid injury to them during pallidotomy.