The results of research of influence of once and repeated fractional external ray treatment of experimental animals in summary dose of 2 Gr on condition of plasmatic membranes of cells in this article are given. Also there are arranged that both once and repeated fractional external ray treatment are accompanied by activation of free-radical peroxidation and, as a result, decrease of content of phospholipids in the matrix of membranes, increase of free fatty acids and products of LPO(lipid peroxidation) and intensification of chemiluminisczantion of suspensions of membranes. The summation of effects of fractional sittings of ray treatment is demonstrated.