This study reports the effects of intraoperative stimulation of the subthalamic nucleus on brain electrical activity in advanced Parkinson's patients. To our knowledge, this is the first study about electroencephalographic responses in the very early phase of deep brain stimulation, during the implantation of the electrodes. We found an increase of gamma band bilaterally over the sensorimotor cortex in the range 45-55 Hz, which was associated with clinical improvement as assessed by means of muscle rigidity decrease. These results indicate that the electroencephalographic gamma responses to deep brain stimulation are present at the very beginning of the treatment process, and may help better understand the short and long-tem effects of deep brain stimulation.