A four-layered cortex is produced in the dorsal region of the rat brain, after 200 cGy X-irradiation at embryonic day 16. The layer 1 is the molecular layer; layer 2 is the upper cellular layer which is in continuity with normal layers V and VI in the lateral cortex; layer 3 is a sparsely cellular layer; and layer 4 is the inner cellular layer which is composed of neurons normally committed to the upper cortical layers. This failure of neuroblast migration to the upper layers is associated with reduced numbers, and morphological abnormalities, of radial glial cells in the dorsal cortex. This further supports the idea that abnormal interactions between neuroblasts and radial glial cells result in abnormal migratory patterns. Since a similar four-layered organization is found in human lissencephaly type I, the present results bring new data that can serve our better understanding of human cortical malformations.