The motor behavior and levels of dopamine and its metabolites in the striatum were studied in rats that received a unilateral injection of 6-OHDA and underwent grafting of rat-derived primary fibroblasts that had been genetically modified to express lacZ and human glial cell line-derived neurotrophic factor (GDNF). Rotation behavior tests were performed each week and striatal levels of DA and its metabolites were measured every 4 weeks after grafting of fibroblasts that expressed lacZ, with or without additional transfection of the GDNF transgene. Rats grafted with GDNF-producing fibroblasts showed a significant improvement in motor behavior as determined by the rotation test, with a less pronounced reduction in the levels of dopamine and its metabolites in the striatum as compared with those in the control animals or brain parts. In addition, there was a lower decrease in the number of TH immunoreactive neurons in the substantia nigra ipsilateral to the lesion in rats with GDNF-producing fibroblasts than in rats with lacZ-expressing fibroblasts. These results support the notion that intracerebral grafting of fibroblasts that express GDNF is a potentially useful therapeutic strategy for treating Parkinson's disease.