The ecological, physiological and species features of the «red» blood system of the red vole Myodes glareolus Schreber, 1780 and the small forest mouse Apodemus uralensis Pallas, 1811 in the adjacent territories of the Kologrivsky and Kostroma districts of the Kostroma region were studied for the first time. It was found that the following indicators: the number of erythrocytes in the blood, hemoglobin content, hematocrit level, erythrocyte diameter of the studied rodents are species-specific and significantly differ in individuals living in the territories of the «Kologrivsky Les» biosphere reserve and the experimental production farm «Minskoe». The correlation of these indicators with gender, year of research, location of capture, proportion of lymphocytes and neutrophils in the blood, body weight of individuals, spleen index, age of rodents, population density and stage of the population cycle was established.