Implanting electrodes for deep brain stimulation (DBS) allows performance of a number of different neurophysiologic studies. Before definitive internalization of the device, recording and stimulation through the electrodes is possible. Stimulation of the globus pallidum produced a mixture of excitation and inhibition of ongoing muscle activity. The excitatory response is probably produced by activation of the large-diameter fibers of the corticospinal tract. Stimulation of the thalamus is mainly associated with EMG inhibition. These findings indicate that the effect of stimulation may vary in different targets and that the mechanism of action of DBS is heterogeneous. A different type of study concerns those undertaken after chronic DBS of the subthalamic nucleus (STN) and globus pallidum internum (GPi) in parkinsonian patients. Cortical mechanisms subserving movement preparation and execution have been assessed. The main findings were that stimulation had a more prominent effect on execution than on preparation and that this was greater in patients treated with STN DBS than GPi DBS.