The site of origin and route and rate of migration of neurons in the developing hippocampal region of the rhesus monkey were studied with tritiated thymidine (3H-TdR) autoradiography. Analysis of specimens sacrificed approximately 1 hour after exposure to 3H-TdR shows that the neurons destined for the hippocampus and subiculum are generated exclusively in the ventricular zone lining the medial wall of the lateral cerebral ventricle. In contrast, neurons of the parahippocampal formation are generated in two proliferative zones: The majority of neurons destined for the lamina principalis interna arise from the ventricular zone, whereas most of those destined for the lamina principalis externa originate from the subventricular zone. The neurons of the dentate gyrus are also generated in two locations: in the ventricular zone (between E38 and E85) and within the prospective hilus of the dentate gyrus (from E58 up to approximately 3 months after birth). Analysis of specimens sacrificed at progressively longer intervals after exposure to 3H-TdR indicates that neurons destined for all of the subdivisions of the hippocampal region (except those cells generated in the hilus of the dentate gyrus) migrate through the intermediate zone, bypassing previously generated neurons on their way to the superficial limits of the developing cortical plate. Estimated migration rates are approximately 15 micrometer/day in the sector of the hippocampal formation, about 100 micrometer/day in the parahippocampal formation, and about 15 micrometer/day in the region of adjacent neocortex. Thus simultaneously generated neurons destined for three distinct cytoarchitectonic areas have significantly different rates of cell migration. These differences are unrelated to the length of cell trajectories and may depend on the mechanism of cell translocation and/or the timing of signals that initiate cell movement. The differential rate of migration indicates that the fate of postmitotic cells may be determined before they have reached their final destination.