We assessed clinical and electrophysiologic characteristics of tremor in patients with parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Four of seven patients with moderate to severe MPTP-induced parkinsonism exhibited a tremor indistinguishable from the characteristic rest tremor of Parkinson's disease (PD). The pathology induced by MPTP in one human case is confined to the substantia nigra, but in nonhuman primates, the locus ceruleus or the ventral tegmental area can also be affected. These findings suggest that the pathophysiology of rest tremor in PD might result from damage to either the substantia nigra alone or in combination with damage to one or more of these other regions.