Using modern ("laser tweezers", low ionic strength Liss, definition of microviscosity of erythrocyte membranes by fluorescence probe pyrene) and classical methods of investigation the osmotic resistance of red blood cells and protein membrane of red blood cells, the authors have esteblisheshed that deformation of the red blood cells in hemic hypoxia varies depending on the animals' age. The maximum capacity of red blood cells to deform was detected in rats of prepubertal period (40.2%). In the group of old rats this parameter reached the minimum level (19.7%). Age changes in erythrocyte membranes during hypoxia are manifested in an increase in the viscosity of the lipid bilayer (microviscosity index reduced in old rats compared with young adult animals by 69.5%) and an increase in the percentage of skeletal protein spectrin (by 53.0%). These changes determine the reduction in deformation capacity of red blood cells. The relationship between the age-related changes of erythrocyte membrane deformation and the state of regional microcirculation has been determined. In old rats a perfusion volume is reduced by 43.8%, the fractional volume of red blood cells passing per unit time through microvessels reduced by 59.3%, the proportion of oxygen consumption in tissues decreased by 6.6% and the perfusion of oxygen saturation decreased by 50.0%.