The inhibitory effects of deltamethrin (DM) on dopamine biosynthesis in PC12 cells were investigated. Levels of dopamine in PC12 cells were detected after treatment with different doses from 10(-4)M to 10(-6)M deltamethrin or combined with 20 microM or 50 microM l-3,4-dihydroxyphenylalanine (L-Dopa) for 3 h or 12 h by HPLC. Flow cytometry (FCM) and RT-PCR technology were used to detect the levels of tyrosine hydroxylase (TH) mRNA and protein in PC12 cells treated with different doses of deltamethrin for 3 h or 12 h. Deltamethrin decreased the levels of dopamine, TH mRNA and TH protein in PC12 cells, and the increased dopamine levels induced by l-Dopa at 20-50 microM were also decreased by deltamethrin for 3 h or 12 h. These results indicate that deltamethrin decreases dopamine content by the inhibition of TH mRNA and protein level in PC12 cells and maybe also have inhibitory effect on aromatic l-amino acid decarboxylase.