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 a newborn, 1‐week, 2‐week, 3‐week, and a 4‐week age condition while the age of sacrifice remained constant at 70 days. Golgi analysis revealed that the interneurons found in the molecular layer of the newborn, 1‐week and 2‐week condition were basket cells. Stellate cells were found in increasing numbers in the 3‐week and 4‐week conditions. Basket cells were found to occupy the entire molecular layer in the newborn and 1‐week conditions resulting in an abnormal axonal plexus. The position of the basket cell soma was found to be in the same plane as its axonal projection. The branching point of the primary dendrite of Purkinje cells was found to correlate with the amount of molecular layer occupied by the abnormal basket cell plexus. Interneurons in the molecular layer were found to show different dendritic growth patterns dependent upon where their soma was located. Interneurons in the deep molecular layer showed only a decreased dendritic field, whereas interneurons in the middle and superficial molecular layer had an abnormal growth of dendrites into the deep molecular layer. A hypothesis is presented to account for the decreased dendritic arborization and the increased length of the primary dendrite before branching, the ectopic basket cell and their abnormally directed dendritic growth, and the difference in behavioral deficits between the newborn and the