Glial cell line-derived neurotrophic factor (GDNF), a member of the TG F-beta superfamily, has been shown to be a highly potent neurotrophic factor that enhances survival of various neuronal cell types including motoneurons. To assess its therapeutic potential in treating neurodegenerative diseases such as amyotrophic lateral sclerosis, we treated mutant mice displaying motoneuron degeneration (progressive motor neuropathy; pmn) with encapsulated GDNF-secreting cells. Effects of GDNF treatment on pmn/pmn mice were compared with previous results obtained with ciliary neurotrophic factor (CNTF) [Sagot Y, Tan SA, Baetge E, Schmalbruch H, Kato AC, Aebischer P (1995) Eur J Neurosci 7:1313–1322]. In contrast to CNTF, GDNF did not increase the lifespan of pmn/pmn mice. However, GDNF significantly reduced the loss of facial motoneurons by 50%, a value similar to what was observed when CNTF was administered to the pmn/pmn mice. Surprisingly, myelinated axon counts revealed that GDNF had no effect on nerve degeneration. Therefore, despite its potential in rescuing motoneuron cell bodies, the inability of GDNF to prevent nerve degeneration in pmn/pmn mice suggests that its usefulness in the treatment of motor neuron diseases may be restricted to cotreatment with other factors that act on the nerve process.