We studied the dynamics of erythrocyte transformation in the blood of 6, 12, and 24-month-old rats after long-term cold exposure (LTCE, 5°C for a month) and short-term (STCE, 9 times at 10°C or –12°C for 2 days). Erythrocyte transformation was assessed by the sphericity index, which was determined from osmotic fragility curves. The percentage of predominant erythrocyte forms was calculated: flattened and normal disco- cytes, spherocytes, and stomatocytes. After LTCE the dynamic of erythrocyte transformation was as follows: 1) in 6-month- old rats, the percentage of discocytes decreased, spherocytes percentage increased almost 5 times; 2) in 12-month-old animals, percentage of flattened highly resistant discocytes rose; 3) in 24-month-old rats, the percentage of both flattened and normal discocytes increased, but percentage of altered forms decreased due to stomatocytes. The STCE (10°C) led to the following changes: 1) in 6-month-old rats, percentage of discocytes waxed due to normal forms, and percentage of altered forms waned due to both stomatocytes and sphero- cytes; 2) in 12-month-old animals, percentage of discocytes increased due to both normal and flattened, and percentage of altered forms significantly decreased due to stomatocytes; 3) in 24-month-old rats, only percentage of altered forms fell. Under STCE (–12°C), changes were as follows: 1) in 6-month-old rats, the percentage of both flattened and normal discocytes decreased, and the percentage of altered forms increased due to spherocytes; 2) in 12-month-old rats, as after LTCE, only the percentage of flattened discocytes enlarged, almost 7 times; 3) in 24-month-old rats, under STCE (–12°C), as after LTCE, the percentage of discocytes rose, and the percentage of altered forms fell due to stomatocytes. The transformation index increased in 6 and 24-month-old rats after all cold exposure regimens, and in old animals several times. In 6-month-old rats, the irreversible transformation index increased sharply under all cooling modes. The same trend was found in 12-month-old animals both after STCE (10°C) and LTCE. No changes were found in 24-month-old rats. The reversible transformation index decreased several times in 6 and 24-month-old animals after all cold exposure modes, as well as in 12-month-old rats after STCE (10°C). Thus, the transformation of erythrocytes depends on the age of rats and the cooling regimens. With age, the heterogeneity of the erythrocyte population in the blood increases (the percentage of discocytes decreases, and, accord- ingly, the percentage of irreversibly changed forms increases). Вікові особливості трансформації еритроцитів у щурів за умов тривалих і короткочасних холодових впливів 109 Cold exposure in 6-month-old rats leads to a decrease in the percentage of discocytes and an increase in altered forms (especially after both regimens of STCE); in 12-month-old (after STCE 10°C) and 24-month-old (after LTCE and STCE -12°C) animals. On the contrary, cold application causes an increase in the percentage of discocytes and a decrease in the percentage of changed forms. That is, after cold exposure in 12-month-old rats and especially 24-month-old rats, the erythrocyte population becomes more homogeneous.