We describe the effect produced by local irradiation with 200 and 700 r on the diurnal rhythm of mitotic activity in the rat corneal epithelium. The effect of total irradiation with 200 r was similar to that of local irradiation with the same dose. However, variation of mitotic activity was not eliminated in this case. After total irradiation with 700 r there was a progressive decline of the diurnal variation. An explanation of these and of published experimental results is put forward.