The brains of young adult rats were irradiated with a single dose of 8 Gy (800 rad) of 250 kVp X-rays. Within 2 weeks of treatment the cell population of the subependymal plate was reduced by 30%. During this period the cell cycle time remained unchanged but the labelling index was reduced. The cell population subsequently returned to normal after 39-52 weeks. Damage and subsequent recovery of the plate was due mainly to changes in the number of cells with small dark nuclei. Cells with small and large light nuclei were little affected. A model for the production and differentiation of cells in the subependymal region is proposed on the basis of age-related changes in the total number and proportions of the various cell types in the subependymal plate of normal rats. This is discussed both in terms of the radiation response of cells in the plate and the manifestation of delayed white matter necrosis after higher doses.