Parkinson's disease (PD) is characterized by the progressive degeneration of the nigrostriatal dopaminergic system. Brain delivery of glial cell line-derived neurotrophic factor (GDNF) has been shown to protect and restore the dopaminergic pathway in various animal models of PD. However, GDNF overexpression in the dopaminergic pathway leads to a time-dependent down-regulation of tyrosine hydroxylase (TH), a key enzyme in dopamine synthesis. In order to elucidate GDNF-mediated biochemical effects on dopaminergic neurons, we overexpressed GDNF in the intact rat striatum using a lentiviral vector-mediated gene transfer technique. Long-term GDNF overexpression led to increased GTP cyclohydrolase I (GTPCH I) activity and tetrahydrobiopterin (BH4) levels. Further, we observed a down-regulation of TH enzyme activity in morphologically intact striatal dopaminergic nerve terminals, as well as a significant decrease of dopamine levels in striatal tissue samples. These results indicate that long-term GDNF delivery is a major factor affecting dopamine biosynthesis via a direct or indirect modulation of TH and GTPCH I and further underscore the importance of assessing both GDNF dose and delivery duration prior to clinical application in order to circumvent potentially adverse pharmacological effects on the biosynthesis of dopamine.