Both long- and short-term contact with the cold environment may cause stable cold adaptation/s. In spite of possible different background mechanisms of such adaptations development, the initial processes are of the same nature and involve autonomic nervous system activation to provide the growing metabolic demands of the body. The later with the corresponding generation of reactive oxygen species and hormonal changes increase the load on the red blood cells (RBCs) and may affect their mechanical stability/shape. The aim of the work was to study the effect of long- and short-term cold adaptation (LTCA and STCA, respectively) on osmotic fragility (OF) and the sphericity index (IS) of RBCs in rats of different ages. White outbred male rats of 6, 12 and 24-month-old were used in the experiments. LTCA was achieved by keeping animals under varying ambient temperature (1-7°C) for 5 weeks under free-running light conditions (food and water ad libitum). For STCA development animals were subjected to -12°C or 10°C environmental temperatures (STCA-12 and STCA+10, respectively) for 2 days (totally 9 cold effects per day) as follows: the first 15 minutes of each hour during daytime the animals were exposed to cold, the next 45 minutes they were left to themselves at 26°C with free access to water and food. RBCs were subjected to hypotonic hemolysis. The resulted OF curves, obtained by the method of small-angle-scattering, were used for determination of the RBCs distribution by the IS, which in turn characterizes the shape of the cells. The shapes of the RBCs that predominated in the certain SI intervals were distributed as follows: (1…1.05) – spherocytes, (1.06…1.5) – stomatocytes, (1.5…2) – normal and (2.1…3) – flattened discocytes. Data were means±SD, compared by ANOVA. No significant changes in OF were found among control and rats after STCA+10 in all age groups. Osmotic fragility decreased (compared to control) in 6 and 24-month-old rats after STCA-12 (from 0.52±0.01 to 0.48±0.01 (p=0.05) and from 0.5±0.01 to 0.47±0.01 (p=0.04), respectively, n=5) and after LTCA in 24-month-old rats (from 0.5±0.01 to 0.46±0.01 (p=0.01), n=6). Moreover, both types of cold adaptations modify RBCs distribution by IS in rats of all ages towards the increase in the percentage of normal and flattened (highly resistant forms) discocytes, and the decrease in the number of cells the shapes of which are close to spherical one (least resistant forms). Thus, under cold pressure the RBCs OF decreased in 6 and 24-month-old animals after STCA-12 as well as in 24-month-old animals RBCs after LTCA. Moreover, both LTCA and STCA improve the RBCs population condition by “washing” out the least resistant RBC forms from the circulation.