Exposure of a monkey retina to the light from a slit lamp for 40 minutes produced a visible retinal change that disappeared after four weeks. Exposures of 20, ten, and five minutes produced no visible changes. Extensive retinal damage was produced in the macula of the other eye of the monkey by a one-hour exposure to the light from an operation microscope. This damage was almost unchanged one year later. Exposure of the monkey retina from the same operation microscope, for the same length of time, with the blue light filtered out, produced a much smaller lesion that, after one year, could not be seen visibly but was detected with fluorescein angiography.