In Parkinson's disease (PD), there is a highly selective loss of dopamine (DA) neurons in the substantia nigra (SN) greater than in the ventral tegmental area (VTA). The simplest explanation for selective DA neuron loss in PD is that DA is toxic and, because only DA neurons contain significant amounts of DA, this highly localized synthesis of DOPAL accounts for selective vulnerability of DA neurons. However, the large concentrations of DA required to produce in vivo toxicity cast doubt on its role in PD pathogenesis. Alpha-synuclein (alpha-syn) is the major component of the Lewy body, the pathological marker of PD, and is genetically linked to the disease. Recent studies indicate that alpha-syn neurotoxicity is mediated by a free radical generating metabolite of DA. Here we test the hypothesis that 3,4-dihydroxyphenylacetaldehyde (DOPAL), the monamine oxidase metabolite of DA, mediates DA toxicity in vivo. We injected DOPAL, DA and its oxidative, reduced and methylated metabolites into rat SN and VTA. Five days post-surgery, the injection sites were evaluated in Nissl preparations and with tyrosine hydroxylase (for DA neurons), neuronal nuclear antigen (for neurons) and glial fibrillary acidic protein (for astrocytes) immunoreactivities. Lesion size in SN vs. VTA was compared using morphometry. DOPAL at concentrations as low as 100 ng was toxic to DA SN neurons>DA VTA neurons>glia. Neither DA nor its other metabolites showed evidence of neurotoxicity at fivefold higher doses. However, 20 microg of DA produced lesions in the SN and VTA. We conclude that DOPAL is the toxic DA metabolite in vivo. Implications for a unified hypothesis for PD pathogenesis are discussed.