Examination of two experimental therapies in patients with advanced Parkinson's Dis. who have become refractory to treatment. Therapy I: IV L-DOPA tests the questions of whether patients with advanced PD will respond to increased hourly intakes of L-DOPA delivered at a constant infusion rate. Non-responders to IV L-DOPA at the maximum infusion rate (120 mg/hr) will then enter the Progabide phase study. In this phase they receive GABA-mimetic drug, Progabide, which augments dopamine activity in the brain.