The retina and retinal pigment epithelium of the anesthetized Rhesus monkey were found to be irreversibly damaged by exposure to the light of a clinical indirect ophthalmoscope for 15 minutes at normal body temperature. Retinal irradiance of the damaging light was estimated to be 0.27 w/sq cm + 20%, a level which is at least two orders of magnitude lower than that associated with solar burns. The morphologic characteristics and natural history of threshold lesions were studied by means of fundus photography, flat preparations of bleached retinal pigment epithelium, and electron microscopy. The temperature rise associated with this exposure was approximately 3.0 C, and the susceptibility of the retinal pigment epithelium was found to be directly related to body temperature during exposure. The precise mechanism responsible for this retinal damage was not ascertained but was felt to be a light effect which was potentiated by heat.