Exposure of rats to 10 to 40 R in fetal or perinatal life produced abnormalities of individual neurons and their organization in the cerebral cortex. The threshold dose was about 10 R just after birth and below 10 R in late fetal life. Perinatally, the most immature neurons of the outer half of the cortex, layers 5 to 2, were most sensitive, and differentiation of their neuclei and cytoplasmic basophllic materials was impaired initially and subsequent dendrlte growth was altered. On the 16th or 18th day of fetal life, radiation initiated changes that led to involvement of all layers of the cortex, a matter of interest because most of the neurons in the cortex did not yet exist except in ancestral precursor form on the 16th day and only a fraction had been "born"by the 18th. There was a persistent retardation of differentiation of all layers of the cortex well into postnatal life after irradiation on the 16th or 18th fetal day. A notable outcome in these fetally irradiated rats was a paucity of neurons in all cortical layers, with a compensatory increase of neuropil such that there was little diminution of cortex volume. This paucity of neurons was attributed to an injury to the source of the neurons, the primitive proliferative-migratory cell system. The nature of the injury which seemed to make the proliferative-migratory cells "forget "how many neurons a cortex should have but not what volume the cortex should occupy is unknown. It suggests, that in producing persistent or delayed changes, radiation may have more specific effects than is often supposed.