The purpose of this clinical trial is to determine whether GDNF works to relieve symptoms of advanced Parkinson's disease. The study will also test the delivery of GDNF using a catheter implanted into the putamen, the area of the brain associated with Parkinson's disease, and an infusion pump that is implanted under the skin in the abdomen or chest. GDNF will be placed into the pump and delivered through the catheter to the brain. The purposes of this study are to determine the potential benefits and side effects of GDNF. The performance and safety of the catheter/infusion pump system will also be assessed. The study will last for 6 months. Subjects will undergo neurological testing, computerized gait assessment and neurological imaging.