Astrocytes engineered to express a tyrosine hydroxylase (TH) transgene are promising potential tools for the treatment of Parkinson's Disease (PD). We previously reported that C6 glioma cells expressing a TH transgene induce behavioural recovery in a rat model of PD, but we did not directly demonstrate the production of either L-3,4-dihydroxyphenylalanine (L-DOPA) or dopamine (DA) by these cells. This point takes on additional interest in view of conflicting reports concerning whether astrocytes engineered to express a TH transgene must be supplemented with the TH cofactor, tetrahydrobiopterin (BH4), for significant L-DOPA synthesis to occur. We have now addressed these questions directly by assaying for L-DOPA and DA production in cultures of the TH-expressing cells. Both L-DOPA and DA were produced by the engineered cells in either the presence of absence of BH4. These results suggest that astrocytes genetically modified to express TH can complete the metabolic pathway from L-tyrosine to DA.