Many isoquinoline alkaloids including berberine lower dopamine content by reducing tyrosine hydroxylase (TH) activity and aggravate L-DOPA-induced cytotoxicity in PC12 cells. In this study, the effects of berberine on 6-hydroxydopamine (6-OHDA)-induced cytotoxicity in PC12 cells and on unilateral 6-OHDA-lesioned rat models were investigated. Berberine at 10-30 ${mu}M$ did not affect cell viability in PC12 cells. However, berberine at concentrations higher than $50{mu}M$ caused cytotoxicity at 24 h. Berberine (10-50 ${mu}M$) also enhanced 6-OHDA (10-50 ${mu}M$)-induced cytotoxicity at 24 h compared to 6-OHDA alone with an apoptotic process. In addition, treatment with berberine (5 and 30 mg/kg, i.p.) for three weeks showed a dopaminergic cell loss in substantia nigra of 6-OHDA-lesioned rats: 30 mg/kg berberine was more intensive cytotoxic. The levels of dopamine were also decreased by berberine (5 and 30 mg/kg) in the ipsilateral substantia nigra of 6-OHDA-lesioned rats. These results suggest that berberine aggravated 6-OHDA-induced cytotoxicity in PC12 cells and treatment with berberine (5 and 30 mg/kg) in 6-OHDA-lesioned rats also enhanced the degeneration of dopaminergic cell death and the decrease in dopamine levels in substantia nigra. Therefore, the long-term L-DOPA therapeutic patients with isoquinoline compounds including berberine may need to be checked for the adverse symptoms.