Epidermal thickness and optical transmission were measured as functions of time in mice exposed daily to UV radiation. The experiments were performed to facilitate an evaluation of the optical influence of epidermal hyperplasia in the process of UV-carcinogenesis. Groups of albino hairless mice were exposed daily to the radiation from fluorescent sunlamps, emitting mainly UVB. With daily doses below the threshold for inducing edema the epidermal thickness increased, while the rate of change diminished gradually with time; the epidermal transmission decreased correspondingly. The speed at which the process progressed in time appeared to be proportional to the daily dose, over a wide dose range. The thickness of the stratum corneum also increased during the experiment. This thickness appeared to be roughly a constant fraction (22%) of the thickness of the whole epidermis. Later in the experiments, a degenerative stage was observed. At this time skin tumors occurred, causing substantial deviations in the epidermal thickness. With daily doses equal to the minimal edema dose, a dramatic acute increase of epidermal thickness was observed.