 The effect of laser treatment to the fundus was studied by several groups. In vivo it could be observed that argon laser photocoagulation of the monkey- and human fundus causes necrosis of the RPE and a detachment of the RPE from Bruch s membrane, budding of individual RPE cells and a multilayered RPE formation in the area of laser irradiation by seven days after treatment. Histologic sections revealed that by irradiating the RPE with a conventional argon laser the whole area of the cells is destroyed and the choriocapillaris as well as the vessels of the choroid are damaged. After laser photocoagulation RPE cells migrate and proliferate to cover the defect. In vivo after mild coagulations as usually performed in macular coagulation the RPE barrier gets intact again. 