Aberrant aggregation of alpha-synuclein (alpha-syn) to form fibrils and insoluble aggregates has been implicated in the pathogenic processes of many neurodegenerative diseases. Despite the dramatic effects of dopamine in inhibiting the formation of alpha-syn fibrils by stabilization of oligomeric intermediates in cell-free systems, no studies have examined the effects of intracellular dopamine on alpha-syn aggregation. To study this process and its association with neurodegeneration, intracellular catechol levels were increased to various levels by expressing different forms of tyrosine hydroxylase, in cells induced to form alpha-syn aggregates. The increase in the steady-state dopamine levels inhibited the formation of alpha-syn aggregates and induced the formation of innocuous oligomeric intermediates. Analysis of transgenic mice expressing the disease-associated A53T mutant alpha-syn revealed the presence of oligomeric alpha-syn in nondegenerating dopaminergic neurons that do contain insoluble alpha-syn. These data indicate that intraneuronal dopamine levels can be a major modulator of alpha-syn aggregation and inclusion formation, with important implications on the selective degeneration of these neurons in Parkinson's disease.