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Seasonal changes in blood erythrocytes and lymphocytes were investigated in Arctic char from a remote high mountain lake at an altitude of 2344 m. Despite striking seasonal differences of environmental conditions the erythrocyte numbers remained constant during the year. Lymphocytes, however, displayed a distinct seasonal pattern with low abundance during the period of ice-cover (about 7 months) and high abundance in summer. During winter the circulating blood contained lymphocytes with damaged organelles and membranes, and almost exclusively mature red cells with a high density. With rising water temperature in July/August (11°C), however, high-density (old) cells almost disappeared, the proportion of immature cells dramatically increased, and the capacity to responding to handling stress decreased. We conclude that in summer the accelerated erythropoiesis (recruitment of red blood cells) compensates for the reduced life span of red blood cells.