The autoxidation and enzymatic catabolism of dopamine results in the generation of reactive oxygen species (ROS), which may possibly contribute to oxidative stress in multiple neurodegenerative disorders. Recent studies indicate that proteasome inhibition occurs in numerous neurodegenerative conditions, possibly as the result of oxidative stress, although the effects of dopamine on proteasome activity have not been determined. In the present study we examined the effects of dopamine on proteasome activity in the neural PC12 cell line. Application of dopamine induced a dose- and time-dependent decrease in proteasome activity, which occurred prior to cell death. Application of an antioxidant (gluthathione monoethyl ester), monoamine oxidase inhibitors (deprenyl, clogyline, paragyline), or an inhibitor of dopamine uptake (nomifensine) attenuated dopamine toxicity and dopamine-induced proteasome impairment. Application of the proteasome inhibitor lactacystin increased the toxicity of dopamine and the levels of protein oxidation following administration of dopamine. Together, these data indicate that dopamine induces proteasome inhibition that is dependent, in part, on ROS and dopamine uptake, and suggest a possible role for proteasome inhibition in dopamine toxicity.