With the use of four independent cell markers and Brd-U birthdating we have charted the earliest stages of neuronal differentiation and migration in the developing rat central nervous system, including the cortex, spinal cord, and retina. One of the markers, the monoclonal antibody 2G12, labeled a large subpopulation of differentiating cells that uniformly lined the ventricles throughout these CNS regions at unexpectedly early ages. Immunocytochemistry demonstrated that, in cortex, the 2G12 antigen could appear in cells during mitosis. More mature looking 2G12-positive cell types initially had a primitive radial morphology and were axonless. However, in strict spatio-temporal sequences, the most mature looking 2G12-positive cells had the ability to sprout GAP-43-positive axons before or after the cell body left the ventricular surface and before or after detachment of their pial or ventricular endfoot processes. Double label experiments with 2G12 and Brd-U showed that none of these three 2G12-positive cell types incorporated Brd-U after a short pulse. The primitive neuroepithelial shape of the immature neurons was verified with a polyclonal GAP-43 antibody, a type III beta tubulin antibody, and DiI labeling from a distal portion of the axon. In the cortex and retina, the 2G12 marker persisted in cells that had reached prospective neuronal layers. However, in all CNS regions observed, 2G12 immunoreactivity disappeared from the cell body as the axon extended from the young neuron. Based on the smooth progression of changing 2G12-positive cell shapes, but also because of the transient nature of this label, we can only speculate that the 2G12 epitope may be marking a continuum of neuronal cell states throughout the earliest period of differentiation and migration. Thus, our hypothesis suggests that many of the youngest CNS neurons may have a widespread distribution and may begin their differentiation, and even remain axonless for a time, while retaining a neuroepithelial morphology. Once differentiation resumes, a major mode of transformation into mature neurons during the earliest stages of development could occur via translocation of the cell soma into the pial process. Importantly, these markers have verified at later stages, and especially in cortex, that multiple mechanisms exist for neuronal migration in the CNS depending on the region and stage of development.