Dopamine (DA) and homovanillic acid (HVA) concentrations were measured in subregions of substantia nigra, ventral tegmental area and retrorubral field in vervet monkeys 1 to 2 months after treatment with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Identical MPTP treatment regimens produced animals with different degrees of parkinsonism. In asymptomatic monkeys, changes in DA and HVA concentrations in the midbrain DA regions were relatively small and involved central substantia nigra and dorsomedial ventral tegmental area. In contrast, changes in symptomatic monkeys were more severe and widespread, significantly affecting all examined subregions of substantia nigra (greater than 75% DA depletion), both dorsomedial and ventromedial ventral tegmental area and lateral, but not medial, retrorubral field. The data indicate that DA neurons in subregions of substantia nigra, ventral tegmental area and retrorubral field are not equally susceptible to MPTP toxicity. The pattern of MPTP-induced DA and HVA losses in the vervet monkey mesostriatal dopaminergic system may resemble postencephalitic Parkinson's disease more closely than idiopathic Parkinson's disease.