Embryonic x irradiation of rat embryos with 150 r at 8.5 and 9.5 gestation days produced a wide variety of maldevelopmerts in the fetuses and post natal animals. Macroscopic variations ranged from exencephaly (herniation of brain) to other severe maiformations including porencephaly, telencephalic defects, and ventricular dysplasias, to undersized (sturted) brains which appeared to be topographically normal. The salient microscopic features of these maldevelopments were various degrees of cytoarchitectural alteration and disorganization associated, at times, with necrobiotic changes. The cellular arrangemerts or patterns of the subependymal or periventricular regions of the lateral and 3rd ventricles often appeared disorganized. Disruption of the ependymal lining was also noticed associated at times with displacement or extrusion of ependymal cells into the ventricular cavity as well as islands of conglomerated cells or groups in rosette formation and random mitotic figures. Electron microscope studies disclosed alteration of the fine structural organization of the nucleus, nuclear membranes, ergastoplasm, Golgi complex, mitochondria and the presence of large numbers of vesicles of the lysosome type. Some blood vessels preserted intimal proliferation associated, at times, with irregular protrusions which, occasionally, appeared detached and within the vascular lumen. The anomalies appear to have no relation to the position of themore » embryos within the uterus, or to the slight differences in the stages of development at the time of x irradiation among the members of the same litter. While genetic factors may be involved, other possible pathogenic mechanisms are to be investigated. A few rat offspring appeared to escape obvious injury from intrauterine 150 r x irradiation at 8.5 and 9.5 g.d. Although they developed into apparertly normal animals, they should not be accepted as truly normal without further studies of their functional capacities.« less