Immunohistochemical examination of the midbrain of vervet monkeys treated with MPTP revealed a marked loss of dopaminergic neurons in the lateral, but not medial, region of the A8 dopamine (DA) cell group. In the same animals, the number of DA neurons in the substantia nigra was only slightly decreased. Biochemical assessment revealed a marked (greater than 85%) depletion of DA in the striatum. However, in the DA cell body regions significant decreases in DA and homovanillic acid were observed only in the lateral A8 region, and not in the medial A8 region or substantia nigra. These data suggest that those A8 DA neurons which project to the striatum are preferentially vulnerable to MPTP.