Although the aberrant expression of α-synuclein (α-Syn) is toxic to dopaminergic neurons, little is known about the correlation between the abnormality of α-Syn and the expression of tyrosine hydroxylase (TH), a rate-limiting enzyme for the synthesis of dopamine neurotransmitter. In this study, the MES23.5 rat dopaminergic cell line transfected with wild-type human α-Syn cDNA (hα-Syn) construct was used to investigate the association between α-Syn and TH. Immunocytochemical staining and Western blot for TH showed that the TH expression was greatly decreased in hα-Syn-transfected cells. Northern blot confirmed that the TH mRNA level was also dramatically reduced. Reduction of TH protein levels did not affect the growth and proliferation of the transfected cells. No apparent cell injury or death was observed. These results suggest that an abnormal expression of α-Syn may inhibit TH synthesis in dopaminergic cells.