Parkinson's disease (PD) patients with prior radio-frequency lesions in the internal segment of the globus pallidus (GPi, pallidotomy), whose symptoms have deteriorated, may be candidates for further invasive treatment such as subthalamic deep brain stimulation (STN DBS). Six patients with prior pallidotomy (five unilaterally; one bilaterally) underwent bilateral STN DBS. The microelectrode recordings (MERs, used intraoperatively for STN verification), ipsilateral and contralateral to pallidotomy, and MERs from 11 matched PD patients who underwent bilateral STN DBS without prior pallidotomy were compared. For each trajectory, average, variance and mean successive difference (MSD, a measure of irregularity) of the root mean square (RMS) of the STN MER were calculated. The RMS in trajectories ipsilateral to pallidotomy showed significant reduction of the mean average and MSD of STN activity when compared with trajectories from patients without prior pallidotomy. The RMS parameters contralateral to pallidotomy tend to lie between those ipsilateral to pallidotomy and those without prior pallidotomy. The average STN power spectral density of oscillatory activity was notably lower ipsilateral to pallidotomy than contralateral, or without prior pallidotomy. The finding that pallidotomy reduces STN activity and changes firing characteristics, in conjunction with the effectiveness of STN DBS despite prior pallidotomy, calls for reappraisal and modification of the current model of the basal ganglia (BG) cortical network. It highlights the critical role of direct projections from the BG to brain-stem structures and suggests a possible GPi–STN reciprocal positive-feedback mechanism.