Microgyria is one type of cortical dysplasia with dyslaminations and anomalous cell densities, which could be produced experimentally. In a normal rat cortical development, overproduced neuroblasts gradually die by apoptosis. Thus, the diminished apoptosis may lead to the anomalous cell densities and dyslaminations. This study examined the ontogeny of cell densities in rat microgyria, in homotopic contralateral cortex and in control cortex. Multiple applications of cold probe (-70degrees C) on the right hemisphere in postnatal day (PN) 0 rat pups produced unilateral microgyria. Cell densities had decreased from PN 1 to 2 in microgyria, in the contralateral left hemisphere, and in the control cortex. From PN3 to PN10, while the cell density of the microgyria had increased (p<0.001), cell densities of the control and homotopic area continued to decrease. Apoptosis was greater in the microgyria than in the control and left hemisphere from PN1 to 3 and then remarkably decreased (p<0.001). These results suggest that the increased cell density in microgyria is not from decreased apoptosis, but probably caused by the increased proliferation or migration.