The amino acid sequence arginine-glycine-aspartate (RGD) acts as a specific ligand for integrins in cell adhesion. We injected RGD into the telencephalic vesicle of mouse embryos exo utero on E13 and examined the cortical histogenesis of the telencephalon on E15,16 and 18. RGD induced disorganized cytoarchitecture and reduced cell number in the cortical plate and intermediate zone. In the ventricular zone although there was no difference in cell density between RGD-injected brains and controls, mitotic activity analysis revealed the lower proliferative rate in RGD-injected brains. These results suggest that RGD-proteins or integrins play an important role both in the migration of neuroblasts and the proliferation of neuroepithelial cells during the cortical histogenesis of the telencephalon.