The effects of harman and norharman on dopamine content and L-DOPA-induced cytotoxicity were investigated in PC12 cells. Harman and norharman decreased the intracellular dopamine content for 24 h. The $IC_{50}$ values of harman and norharman were 20.4 ${mu}M$ and 95.8 ${mu}M$, respectively. Tyrosine hydroxylase (TH) activity and TH mRNA levels were also decreased by 20 ${mu}M$ harman and 100 ${mu}M$ norharman. Under the same conditions, the intracellular cyclic AMP levels were decreased by harman and norharman. In addition, harman and norharman at concentrations higher than 80 ${mu}M$ and 150 ${mu}M$ caused cytotoxicity at 24 h in PC12 cells. Non-cytotoxic ranges of 10-30 ${mu}M$ harman and 50-150 ${mu}M$ norharman inhibited L-DOPA (20-50 ${mu}M$)-induced increases of dopamine content at 24 h. Harman at 20-150 ${mu}M$ and norharman at 100-300 ${mu}M$ also enhanced LDOPA (20-100 ${mu}M$)-induced cytotoxicity at 24 h. These results suggest that harman and norharman decrease dopamine content by reducing TH activity and aggravate L-DOPA-induced cytotoxicity in PC12 cells.