After X-irradiation with 200 rd on day 12 of gestation in the mouse, many typical cell clusters (rosettes) presumably originating from the ependymal layer can be observed in the central nervous system on day 18 p.c. In addition, many necrotic areas are found close to the ventricular surface in the subependymal layer, which is the main place of reparative cell proliferation. Typically, glial cells are arranged radially around these necrotic areas, in so doing druse-like structures which can be regarded as a peculiarity of the reparative process. Only macrophages and no astrocytal cell reactions are to be found. Immediately after birth the rosettes, as well as the druse-like structures, begin to collapse. The rosettes appear to be more marked off from their surroundings and many of them are in a degenerative state. Those situated near the ventricular surface form vaults with distinct dequamative properties. The outcome of this atrophy is a flattening of the cortical wall and the appearance of smooth-walled cysts, predominantly at the lateral angles of the telencephalic walls. No reactive process, especially no glial scarring, can be observed. This cyst formation proceeds after birth in a centrifugal direction, until clear porencephaly evolvement becomes noticeable. It is concluded that these peculiar reparative morphological changes are connected with the drastic physiological alterations which occur at birth. Fetal X-irradiation may be followed by a metabolic disorder, which leads to the definite histological effect of porencephaly only after birth, when metabolic imbalances are most critical.