AbstractThe whole‐head of infant kittens was irradiated with fractionated doses of 150 R and 200 R at different postnatal intervals. Experimental age conditions consisted of an irradiated newborn, 1‐week, 2‐week, 3‐week, and a 4‐week age condition while the age of sacrifice remained constant at 70 days. The molecular layer thickness was reduced by 47% in the newborn, 40% in the 1‐week group, 17% in the 2‐week group, 19% in the 3‐week group and by 9% in the 4‐week group. An evaluation of Golgi impregnated material revealed that the dendritic arborizations of Purkinje cells were consistently reduced the earlier the age at which radiation was begun. A reduction in spiny branchlets was seen in all of the experimental conditions. All experimental age conditions displayed the phenomenon of dendritic “damming” at the pial surface, which consisted of an excessive crowding of spiny branches at this level. An increased growth of the primary dendrite before branching tood place was seen in the newborn and 1‐week group. This became less but was still significant in the 2‐week group. A correlation of this phenomenon with the presence or lack of stellate cells is discussed. Purkinje cells with two or more dendrites emerging from their soma were classified and analyzed separately. It was found that the primary dendritic branching in these cells often followed separate morphological patterns and appeared to be independent of each other. Climbing fibers were found to conform to the abnormal dendritic arborizations of the Purkinje cells, and were reduced in complexity in the early radiation treatment groups. This suggested that climbing fibers had no influence upon the dendritic growth pattern, but instead were under the influence of the Purki