Adult rhesus and pigtail monkeys were exposed to a uniform field of light from daylight fluorescent lamps to determine the initial site and characteristics of structural retinal damage induced by continuous exposure and the threshold intensity required to produce the effects. Electron microscopic examination revealed that the initial site of damage is the photoreceptor outer segments. Damage remained restricted to the outer segments at intensities as high as 24,700 lux and with 12 hr exposure periods repeated for up to 4 days. Rods were swollen at the distal tip and showed disc membrane separation. Come outer segments were affected at the proximal end with vesiculation and membrane rearrangement. The threshold intensity for morphological changes to comes in the macula for a single 12 hr exposure was between 195 and 361 mu W/cm2 at the retina (400 to 700 mm; uncorrected for ocular transmittance), whereas the threshold for changes in rods was higher, at between 361 and 615 mu W/cm2. These levels correspond to between 5900 and 10,800 lux (550 to 1000 ft-cd) for comes and 10,800 to 19,400 lux (1000 to 1800 ft-cd) for rods of monkeys with fully dilated pupils. The paramacular areas of the retina were less sensitive to damage than macular areas of the same animal. No paramacular changes were observed in animals exposed to 10,800 lux or less. The patched (control) eyes of each monkey remained structurally normal in both the macula and paramacula at all exposure levels. These results demonstrate the sensitivity of the adult primate retina to damage by relatively moderate levels of light.