The aim of the present study was investigation of the lipid peroxidation changes within the erythrocytes under conditions of increased RBC aggregation. This latter was produced both in the in vitro and in vivo conditions by the addition of Dextran T-500. For the in vitro studies blood samples were taken from the cubital veins of 15 healthy subjects. During the in vivo studies 10 ml of the 10 percent Dextran T-500 solution was administered intravenously in six chinchilla rabbits. Another six animals were treated with rheopolyglucyne. The RBC aggregation in blood was investigated with the "Georgian technique". The malondialdehyde (the end product of lipid peroxidation) was determined in all cases by its reaction with thiobarbituric acid. We found that in the in vitro conditions, as well as in the in vivo studies, the lipid peroxidation was significantly increased in the erythrocytes during the enhanced RBC aggregation induced by addition of Dextran T-500. Therefore we suppose that the elevated RBC aggregation cause an increase of cell's lipid peroxidation and it is possible to think that appropriate prooxidant: antioxidant balance is shifted toward the pro-oxidants in the erythrocytes.