High-frequency electrical stimulation of the internal pallidum (GPi) or the subthalamic nucleus (STN) improves clinical symptoms of Parkinson's disease. In 12 parkinsonian patients, 6 with STN and 6 with GPi stimulators, we used H215O positron emission tomography to evaluate whether changes in movement performance were accompanied by change in regional cerebral blood flow (rCBF). Patients were scanned both at rest and while performing a free-choice joystick movement, under conditions of effective and ineffective electrostimulation. During effective STN stimulation, movement-related increases in rCBF were significantly higher in supplementary motor area, cingulate cortex, and dorsolateral prefrontal cortex (DLPFC) than during ineffective stimulation. No significant change was observed in any of these areas during GPi stimulation. The difference between the effect of STN and GPi stimulation on movement-related activity was mainly localized to DLPFC. These results confirm the dominant role of nonprimary motor areas in the control of movement in parkinsonian patients and demonstraste the importance of STN input in the control of these areas.