Mechanical removal of the horny layer immediately after UV irradiation of the skin did not result in development of erythema when irradiated in the minimal erythemal dose (MED). When a patient was irradiated in a dose of 2 MED, the intensity of erythema was drastically reduced. This leads to a conclusion that the metabolites accumulating on the surface of the skin and in the horny layer during their oxidation with UV light are capable of return diffusion into the skin where they act as erythema mediators. Simultaneous recording of lipid peroxidation and oxygen consumption in the removed horny layer has demonstrated the relationship between the intensity of oxidation processes in the horny layer and the UV irradiation dose.