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A complex of erythropoietic parameters has been studied for the first time in a pygmy wood mouse Apodemus uralensis Pall., which is widespread in the Caucasus Mountains. Studies were carried out on adult animals captured in winter, spring, summer and fall in a pine forest (Elbrus National Park, The Kabardino-Balkarian Republic) at the height of about 1800 m above sea level. The seasonal dynamics of bone marrow erythropoiesis, the release of young erythrocytes (reticulocytes) into peripheral blood, and the peculiarities of maintaining optimal blood oxygen capacity (BOC) during different seasons of the year under conditions of chronic oxygen deficiency were studied. Bone marrow erythropoiesis in A. uralensis is maintained at a high level throughout the annual cycle, while being most active in winter and summer compared to spring and fall. Living under conditions of chronic hypoxia necessitates a high oxygen supply to the organism of A. uralensis regardless of the season. However, in different seasons, BOC is maintained in different ways. During a cold fall–winter period, this is provided by an increase in the total cell surface area for oxygen absorption, whereas in a spring–summer period, this is achieved by a release of erythrocytes with qualitatively different properties into the bloodstream. The revealed seasonal variability in the parameters of the A. uralensis erythron ensures the maintenance of both the optimal BOC and whole-organism homeostasis under mid-mountain conditions of the Central Caucasus. [ABSTRACT FROM AUTHOR]