Cellular composition of blood was studied in vessels of reproductive organs and jugular vein in females of reindeer and cow depending on their physiological state. Physiological leukocytosis was revealed in reproductive organs of the animals, the most expressed in reindeer. In both species, during the estrous cycle, in blood of vessels of reproductive organs the content of lymphocytes and monocytes is higher than that of granulocytes. With onset of pregnancy, in vessels of ovary and uterus in reindeer and cow the number of lymphocytes and monocytes decreases, while the number of granulocytes increases due to a rise of eosinophils and basophils. The more successful reproduction of reindeer females under conditions of North seems to be owing to an increased immune reactivity of reproductive organs.