Prior to the emergence of the major functional subdivisions of the mammalian forebrain--the neocortex, hippocampus, olfactory bulb, basal ganglia, and basal forebrain--the lateral aspect of the telencephalic vesicle is distinguished by early neuronal differentiation assessed by MAP2 and GAP43 expression and increased expression of the Ca(2+)-independent/immunoglobulin superfamily cell adhesion molecules (CAMs) NCAM, L1, and TAG-1. In contrast, the ventral and medial aspects of the vesicle show little early neuronal differentiation and intermediate or undetectable levels of CAM expression. We asked whether cells from these three regions acquire distinct adhesive and recognition properties that reflect their position, state of neuronal differentiation, and level of CAM expression. In a dissociation/reaggregation assay, cells from the lateral telencephalic vesicle form the largest reaggregates while ventral reaggregates are of intermediate size and medial reaggregates are the smallest. This differential adhesion has a Ca(2+)-independent component, and cells in reaggregates from each region maintain expression of CAMs and other neuronal markers consistent with their region of origin. Furthermore, cells from the lateral telencephalon can specifically sort out from medial cells. Little adhesivity is observed prior to early neuronal differentiation and the expression of Ca(2+)-independent CAMs, when the forebrain is still a prosencephalic vesicle, nor does it follow the pattern of detectable CAM expression once forebrain rudiments are formed. Thus, cells in the early developing forebrain acquire distinct adhesive and recognition properties that reflect the concurrent emergence of regional differences in neuronal differentiation and CAM expression. These differences are transient and can only be detected in the telencephalic vesicle before and during the morphogenesis of rudiments of major forebrain subdivisions.