Although cataract production has not been established as a microwave effect in humans,* the formation of lens opacities in animals subjected to 12.3 cm. radiation † suggests the need for precautions in the administration of ocular diathermy or in situations where exposure of the eye is required for the operation of microwave generators.‡ Unfortunately, the lack of basic information as to the radiation dose and dose rates employed in previous studies has thus far precluded the development of standards which define ocular tolerance to microwaves. As an initial effort in this direction, the present investigation is designed to establish the time and power requirements for experimental opacity production by single doses of 12.3 cm. radiation. Both the wave length and the power levels employed for this purpose are of interest in the applied fields of microwave diathermy, radar communications, and microwave research. The chief source of information on lens damage