The effects of ionizing radiation on the crystalline lens have received intensive study in the past decade; however, radiation studies on the retina have not been emphasized In an earlier investigation Co60gamma radiation was delivered to the head region of young adult monkeys. Pyknosis of the visual cells occurred with doses as low as 2000 r; however, no attempt was made to establish a time threshold for the appearance of retinal abnormalities. It is the purpose of the present paper to describe the retinal changes found in a study of 48 monkeys which had been exposed to massive doses of Co60gamma radiation delivered at a high dose rate. Our aim in this investigation was the determination of a time threshold for pyknosis of visual cells following a single dose of ionizing radiation, and also to ascertain whether this was a direct radiation effect. Fixed doses of