The regionalization of the cerebral cortex proceeds gradually from early embryonic stages under the control of transcription factors that are expressed in gradients. Two phases can be distinguished at the beginning of cortical development: the genesis of a precocious and transient structure, the preplate, which is followed by development of the cortical plate within the preplate. Cellular indices of early regionalization have not yet been described either in the preplate or in the early cortical plate. In the present study, we identify two regions, lateral and dorsal, in the mouse cortex embryo, which differ strongly in the functional properties of their early neurons. By using culture experiments and grafts on organotypic slices, we show that the earliest neurons in the dorsal cortex extend axons before and more rapidly than the earliest neurons in the lateral cortex. In contrast to the lateral cortex, the dorsal cortex differentiates neurons migrating along axons in vitro. These cells express markers of the GABAergic lineage. Early differences between the two regions suggest that the dorsal part of the cortex generates early neurons with particular intrinsic properties that may in turn specifically influence the later development of the cortical plate in this domain.