The distribution and morphology of serotoninergic neurons in the marmoset (New-World monkey) brainstem were studied by immunocytochemistry and computer-assisted three-dimensional reconstruction. The cytoarchitectonic localization of serotoninergic neurons was ascertained in series of adjacent immunostained and Nissl-stained sections, and the extent and shape of the serotoninergic nuclei were visualized by computer reconstruction. The overall distribution of the immunoreactive neurons is comparable to that already described for several species of primates. The serotoninergic nuclei are spatially well segregated into an anterior and a posterior group. The anterior group, in the mesencephalon and the rostral pons, contains the largest population of serotoninergic neurons. These neurons are not confined to the raphe nuclei near the midline, but rather expand laterally in the reticular formation. This expanded distribution of the neurons in the anterior group results in a partial fusion of the nuclei. In some nuclei, particularly the median raphe, subdivisions can be clearly delineated on the basis of the distinct morphology of the neurons and of their clustering. The neurons of the posterior group, in the caudal pons and the medulla, are almost all contained within the limits of the raphe nuclei. The serotoninergic neurons located in the reticular formation form a lateral column, which is clearly separated from the serotoninergic neurons found near the midline. Immunoreactive axons are distributed throughout the brainstem, but they innervate certain motor and sensory nuclei more densely. It was consistently found in newborn animals that the overall immunoreactive axonal network was richer than in juveniles or adults, suggesting that there may be a major modification in the function of the serotoninergic system around birth.