In experiments in vivo the effect of nanocerium (cerium oxide nanoparticles) on the stability of red blood cells to acid hemolysis, levels of both ROS and RNS generation and H2S pools in plasma and erythrocytes of old rats were investigated. In red blood cells of old rats the proton penetration into the matrix of erythrocytes showed a significant raising and the fate of labile «aging» erythrocytes in old animals compared with adult were upregulated. These phenomena paralleled with significant up-regulation of ROS and RNS generation. Introduction for 14 days per os to old rats 0.1 mg / kg of nanocerium fully restored resistance of erythrocytes to acid hemolysis by ROS and RNS in both plasma and erythrocytes reduction. Nanocerium decreased the erythrocytes and, conversely, significantly increased the plasma’s pools of H₂S. Key words: erythrocytes, acid hemolysis, old rats, proton penetration, cerium oxide nanoparticles, reactive oxygen and nitrogen metabolites, H₂S.