[en] Rat cerebellum was irradiated with 100 R daily doses from birth to 10 days of age, and the animals were studied during the next 13 days. The growth of the body and of the forebrain were little affected, but that of the cerebellum was severely retarded. This was primarily due to a depression in new cell acquisition which during the irradiation period was only about 10 percent of that in the controls. On the other hand, it seems that the development of cells formed prior to irradiation was little affected; at day 10, the average size and the RNA and protein contents of the cells were significantly higher than at birth and they were more than double the values observed in the control. However, cell formation was not irreversibly affected: in the fortnight after the termination of irradiation the rise in cell numbers was more than 80 percent of that occurring in the control rats. A relatively normal development of the cerebellar cortex was indicated by the finding that the molecular and the internal granular layers increased substantially in size during the postirradiation period. Further, by 23 days of age the external granular layer, which is a main germinal site in the cerebellum disappeared, as in controls, and the concentration of DNA (packing density of cells) and the cellular contents of RNA and protein were normal. However, restitution was not complete: at 23 days of age, in comparison with controls, the weight of the cerebellum was 60 percent and the reduction in the total number of cells (-40 percent) was similar to the reduction in size of the internal granular layer, which contains the highest concentration of nerve cells in the cerebellum. (U.S.)