Since the recognition that the compound l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP) is a potent inducer of chronic and irreversible parkinsonism in humans, it has been the object of intense and extensive research activity. The fact that MPTP selectively kills cells in the pars compacta of the substantia nigra and produces a syndrome in human and nonhuman primates that is virtually indistinguishable from the idiopathic disease has led to hope that understanding the mechanism of action of this toxin may shed light on the underlying biochemical pathology that is responsible for cell death in Parkinson's disease. A brief overview of Parkinson's disease is presented, and current data regarding the effects, biotransformation, and mechanism of action of MPTP are discussed.