The anterior portion of the head was irradiated in 70 adult axolotls (Mexican salamanders of the species Siredon mexicanum), 4–5 years old, using 1000 r in 9 animals, 3000 r in 20, 6000 r in 10, 8000 r in 11, and 10,000 r in 20; 5 animals served as untreated controls. Specimens fixed at various intervals following irradiation were studied histologically. After irradiation with 1000 r, the substantia propria of the cornea was almost normal. The corneal epithelium, however, became much thicker, and melanophores and Leydig cells developed in it. After irradiation with 3000 r, the substantia propria was much thicker than normal, and underwent stratification; cavities and pigment cells developed in it. In the epithelium, melanophores and Leydig cells developed, and sometimes cavities formed between the cells, and vacuoles appeared in the cytoplasm. After irradiation with 6000–10,000 r, the substantia propria underwent thickening and stratification, and blood vessels penetrated into it. The epithelium became much thicker, giant cells developed, and melanophores and Leydig cells were formed. Sometimes the corneal epithelium disappeared and was replaced by skin epithelium. Some phenomena typical of the response of young axolotls to radiation (e.g., the formation of abnormal outgrowths on the cornea) were not observed in adult animals. On the other hand, the development of melanophores and blood vessels in the substantia propria occurred in the adults, but not in the young animals. The development of melanophores and blood vessels in areas of the irradiated cornea where they do not occur in the nonirradiated cornea may be considered a result of X-ray stimulation. Most of the histopathological changes that were observed in the irradiated cornea in axolotls are not peculiar to amphibians; they have been reported by various investigators to occur in mammals, including man.