Abstract <p>Variability and diversity of adaptations to environmental hypoxic conditions is determined by both its duration and the presence or absence of concomitant hypercapnia. One of the key parameters determining these adaptations is the oxygen-carrying capacity of blood, which is driven by hemoglobin content. However, the ecological patterns of variability of hematological parameters that characterize oxygen-carrying capacity of blood are not clear. Multivariate analysis of the red cell content in blood pictures of 12 species of muroid rodents of various ecological specializations distinguished three main factors determining its variability. The first factor is determined by the number of red cells, amount of hemoglobin, and hematocrit; the second factor is determined by the volume of red cells and their content of hemoglobin; and the third, by the average concentration of hemoglobin in a single red cell. In the northern and Zaisan mole voles, specialized to subterranean life, and the burrowing Djungarian hamster, the volume of erythrocytes and their hemoglobin content are significantly higher than in the mountain voles that inhabit high altitude habitats. It can be assumed that the formation of structural adaptations to hypoxia depends on whether it is accompanied by hypercapnia (as in burrowing species) or not (as in the mountain species). At the same time, a significant range of variability of the considered indicators in ecologically and taxonomically close species does not allow us to identify its general evolutionary patterns on the available material.</p>