Until quite recently a major puzzle in respect to the pathogenesis of radiation cataract has been the long lag period between exposure and the first visible signs of opacification in the lens. Von Sallmann * has shown that mitotic and other nuclear injury in the lens epithelium is an immediate effect of exposure to ionizing radiation, and Cogan3 has found that these injured epithelial cells migrate posteriorly under the lens capsule, thus causing the posterior subcapsular disc-shaped opacity which is so characteristic of radiation cataract. There remains, however, a very large gap in our knowledge, for we have as yet no firm grounds for connecting these epithelial cell changes with the subsequent damage to lens fibers and the development of cortical opacities in the lens. The present study is an attempt to narrow this gap by seeking the earliest recognizable changes in the lens fibers. The study of the histopathology