The 1-methyl-4-phenyl-pyridinium ion (MPP+) is the four electron oxidation product of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6 -tetrahydropyridine (MPTP). MPP+ can be formed by the oxidation of MPTP by monoamine oxidase B to the intermediate dihydropyridinium species, MPDP+, which is spontaneously transformed to MPP+. In the present study, MPP+, like the mitochondrial toxin rotenone, inhibited pyruvate-malate respiration in isolated mitochondrial preparations. Moreover, the stereotaxic administration of both MPP+ and rotenone caused damage to the dopaminergic nigrostriatal pathway. These data clearly demonstrate that a mitochondrial toxin, administered stereotaxically, is extremely neurotoxic. The data lend support to the concept that MPTP-induced neurotoxicity may be due to the detrimental actions of enzymatically formed MPP+ on mitochondrial function.