Morphological changes in ventral mesencephalic dopamine (DA) neurons of a monkey sustaining a unilateral electrolytic lesion of the ventromedial mesencephalic tegmentum four years earlier were examined. Substantia nigra (A9) DA neurons lateral to the lesion underwent hypertrophic changes. The mean area of these neurons was enlarged by approximately 30% relative to corresponding neurons in the contralateral substantia nigra. Semi-quantitative immunohistochemical measurements of the intensity of tyrosine hydroxylase-like immunoreactivity (TH-li) indicated an increase in the amount of TH-li protein per cell in the hypertrophied neurons. Hypertrophic changes were also observed in ipsilateral A11 DA neurons of the caudal hypothalamus, suggesting that the increase in size was related to transection of the axons of DA neurons as they pass through the midbrain in their projections to target sites. The lesion did not overtly change the density or pattern of the substance P innervation of the substantia nigra, indicating that the striato- and pallido-nigral projections were spared by the lesion. These data suggest that hypertrophy may be a compensatory mechanism of dopaminergic neurons in response to partial lesions of the nigrostriatal system, and thus represent a morphological counterpart to the compensatory biochemical processes effected in response to partial lesions of the striatal dopaminergic innervation.