Parkinson's disease is characterized by loss of neurons that produce dopamine in a region of the brain called the substantia nigra. In the early stages of the disease, the disease responds to agents that replace dopamine such as levodopa. Patients with more advanced disease have wide fluctuations in their response to levodopa, exhibiting on and off periods. This is due to continued degeneration of neurons. Recombinant-methionyl human glial cell line-derived neurotrophic factor (r-metHuGDNF or GDNF) is a neurotrophic factor that promotes survival of dopaminergic neurons. This is a protein produced by recombinant technology that is almost identical to the naturally produced factor. Results of animal studies indicate that GDNF has the potential to benefit patients with advanced Parkinson's disease.