The neonate rats of 5 days postnatal age were exposed to low-level X-ray irradiation or were administered microquantities of N-ethyl-N-nitrosourea directly into the cerebellum. Following the initial phase of cell degeneration and phagocytosis of the debris, the surviving Purkinje cells showed recovery of their differentiation. During this stage some Purkinje cells were observed to undergo mitosis, and they were identified in light- and electron-microscopic preparations. The relationship between tetraploidy in the Purkinje cells during their postnatal differentiation and induction of mitosis in them by experimental means is discussed.