Chlorpyrifos (CPF) is one of the organophosphorus pesticides widely used around the world, especially in China. Acetylcholinesterase inhibition is the main effect of organophosphorus insecticides exposure. Studies showed that CPF may also interfere with the metabolism of monoamine transmitters. To investigate the effects of CPF on dopaminergic pathway, the dopamine content, gene expression of catechol-O-methyl- transferase (COMT), vesicular monoamine transporter-2 (VMAT-2), and monoamine oxidase (MAO) and its activity in PC12 cells exposed to CPF was determined. Results showed that cell viability was decreased and total dopamine concentration was increased with CPF administration in a dose-dependent pattern. Gene of MAO was significantly downregulated in PC12 cells, while genes of COMT and VMAT-2 in PC12 cells did not show any change after CPF exposure. The MAO activity was decreased following incubation exposed to CPF. These results suggest that CPF may interfere with dopaminergic pathway through inhibition on gene and protein expression of MAO in vitro.