Mouse isocortical cells were dissociated at 18 days of embryonic development and were reaggregated in vitro by rotation in gyratory incubator shaker. The internal organization of the resulting aggregates was studied by conventional light microscopy, Golgi impregnation, and electron microscopy, establishing the following pattern of reassembly: (1) the predominant cell type in the aggregates was the pyramidal neuron; (2) each of these pyramidal neurons tended to orient its apical dendrite toward the surface of the aggregate; and (3) in larger aggregates (diameter > 600 microns) there was prominent parallel alignment of pyramidal cell apical dendrites. These characteristics resulted in an in vitro reconstruction of the major features of isocortex observed in situ, including formation of a superficial, rather acellular plexiform layer. Reconstruction of isocortical architecture appeared to take place independently of either a germinal epithelium, a radial glial framework, or an outer mesenchymal scaffold. Analysis of the events occurring during aggregate formation suggests that intrinsic cellular information accounts for the expression of basic pyramidal cell morphology. However, dendritic orientation and alignment are most likely determined by cell-cell interactions dependent upon specific cell surface recognition properties, as well as by geometric restraints imposed by the spherical or cylindrical shape of the aggregates.