The compound 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induces an irreversible neurological syndrome in man and monkey which is similar to idiopathic Parkinson's disease in its clinical, pathological, neurochemical and pharmacological response properties. MPTP is selectively neurotoxic to the dopaminergic regions of the brain, destroying neurones in the substantia nigra (A8 and A9 cells, nigrostriatal system) but not the ventral tegmental area (A10 cells, mesolimbic system). Selective dopamine depletion and nigral cell loss after MPTP treatment has also been reported recently in the mouse. The mechanism by which a peripherally administered, low-molecular weight compound exerts permanent but selective toxic effects on dopamine systems in the brain may be relevant to parkinsonian syndromes induced by other toxins and to the disease process in idiopathic Parkinson's disease. We report here that MPTP is oxidized in the brain to a pyridinium species (a compound with potent herbicidal activity) and, in the monkey, is trapped intraneuronally. Furthermore, we demonstrate that this enzymatic oxidation is blocked in vivo in the mouse by a monoamine oxidase inhibitor, a condition which also blocks the neurotoxicity, indicating that the oxidative metabolism of MPTP is required for its neurotoxic effect.