Glial cell line-derived neurotrophic factor (GDNF) shows potent and relatively specific protective effects on dopaminergic neurons. However, the size of the GDNF protein (MW 32-42 kDa) precludes the clinical use of GDNF via parenteral administration. It would thus be useful to have a cell line that stably secretes GDNF with full biological activities. The present study shows that NIH 3T3 cells express a considerable amount of GDNF. After co-culturing with primary E14-E16 midbrain neurons, such cells protected primary rat midbrain TH-immunopositive neurons from degeneration and MPP+ toxicity. In order to enhance endogenous GDNF expression, NIH 3T3 cells were stably transfected with GDNF cDNA with the Kozak sequence. The clones with the highest GDNF expression level were selected. The protective effects of engineered cells increased as the GDNF expression level increased. These cell lines may merit clinical investigation.