LESIONS were placed in the brain-stem of 19 monkeys. They were killed following a post-operative period of one to nine months. The brain-stem was kept for the microscopic study of the lesions. The lenticular nucleus (mostly putamen) and caudate nucleus were dissected out from both sides for the determination of their catecholamine content. In one animal the lesion was bilateral. In 6 animals the unilateral lesion, located in the ventromedial tegmental area, caused a severe loss of cells in the pars compacta of the ipsilateral substantia nigra and was associated with a low concentration of dopamine and of norepinephrine in the corresponding striatum (18 to 42 per cent of the concentration of the intact side). In 12 animals the tegmental lesion which did not affect the cellularity of the substantia nigra did not significantly alter the concentration of the same catecholamines in the corresponding striatum (71 to 130 per cent of the concentration of the intact side). The results suggest that the pars compacta of the substantia nigra normally exerts through its efferent nervous pathways a direct influence on the catecholamine concentration of the corresponding striatum.