Morphologic changes characteristic of mild and severe cell injury produced in retinal photoreceptors by a xenon arc coagulator were studied by light and electron microscopy. Mild or reactive cell changes consisting of a tubulovesicular rearrangement or breakdown of cell membranes differed considerably from the production of myriad focal densifications of cell membranes and associated ground substances occurring in the severe injuries. The latter alterations are considered to beirreversible and, therefore, a characteristic of necrosis.