In this study the development of serotonergic (5-HT) neurons is followed from their initiation of transmitter synthesis until the establishment of an essentially mature morphology. We have used the new and sensitive technique of 5-HT immunocytochemistry to visualize the precise features of this process. The great stability of this method, and the feasibility of counter-staining tissue sections permits the visualization of dendritic processes and axon terminals, as well as perikarya, and facilitates the localization of these structures with respect to non-5-HT components of the neuropil. Serial transverse and sagittal sections of rat fetuses on embryonic days (ED) 13, 14, 15, 17, 19 and 21, and postnatal rats on days 1, 3, 4 and 10 were examined. A detailed photomicrographic map showing the locations of 5-HT neurons at all prenatal stages is provided. The development of 5-HT neurons is evaluated in terms of their cellular morphology, particularly dendritic architecture, the relationship of these cells to the development of the surrounding brainstem, and the morphology and packing density of the 5-HT nuclei. From these considerations a model is proposed of the pattern of cell migration within the nuclei that give rise to the ascending 5-HT projections. At E14 a relatively simple configuration of bilateral superior (rostral) and inferior (caudal) 5-HT cell groups is present. In the period extending from E14 to E19 several subgroupings of these cells develop, presumably as the result of differential cell migration. Based on the predominant dendritic orientation of these cells it is possible to reconstruct their probable migratory paths. At E19 the 5-HT neurons are distributed in groups that are similar to those seen in the adult. In the time from E19 until the end of the first postnatal week there is rapid growth of 5-HT dendrites and a marked decrease in cellular packing density. These alterations shape the nuclear aggregates into the form seen in the adult. The development of the 5-HT cell groups is discussed in the context of known features of neurogenesis, migration, and axonal projections of the raphe and medial reticular nuclei of the brainstem. The possibility is raised that the decrease in cellular packing density in the 5-HT nuclei may reflect the appearance of the non-5-HT components of the raphe nuclei.