With a view toward improving delivery of exogenous glial cell line-derived neurotrophic factor (GDNF) to CNS motor neurons in vivo, we evaluated the bioavailability and pharmacological activity of a recombinant GDNF:tetanus toxin C-fragment fusion protein in mouse CNS. Following intramuscular injection, GDNF:TTC but not recombinant GDNF (rGDNF) produced strong GDNF immunostaining within ventral horn cells of the spinal cord. Intrathecal infusion of GDNF:TTC resulted in tissue concentrations of GDNF in lumbar spinal cord that were at least 150-fold higher than those in mice treated with rGDNF. While levels of immunoreactive choline acetyltransferase and GFRalpha-1 in lumbar cord were not altered significantly by intrathecal infusion of rGNDF, GDNF:TTC, or TTC, only rGDNF and GDNF:TTC caused significant weight loss following intracerebroventricular infusion. These studies indicate that insect cell-derived GDNF:TTC retains its bi-functional activity in mammalian CNS in vivo and improves delivery of GDNF to spinal cord following intramuscular- or intrathecal administration.