Selenium is an essential trace element in the diet of farm animals. Selenium is a part of the antioxidant enzymes, preventing the free radicals cell destruction. Selenium availability varies greatly due to the great number of biogeochemical territories. The NorthWestern region of Russia is a territory with selenium deficiency in soils and plants. In this study, it was found that the concentration of selenium in pasture plants was in the range from 0,059 to 0,067 µg /kg with an optimal value of 0,1 mg/kg. It was found that the concentration of hemoglobin, the number of red blood cells, the concentration of total protein and albumins in blood of Romanov sheep over the period of lactation was below the reference values. The number of red blood cells also increased. On the 100th day, the number of red blood cells was higher by 7.78% compared to the 30th day and by 14.59% compared to the 10th day since the beginning of lactation. There was an increase in the concentration of total protein due to the albumin fraction. On the 100th day, the total protein concentration was 15.29% higher compared to the 10th day and 9.46% higher compared to the 30th day since the beginning of lactation. On the 100th day from the start of lactation, the hemoglobin concentration was 19.07% higher compared to the 10th day from the start of lactation and 10.73% higher compared to the 30th day from the start of lactation. It was marked that those parameters had got higher to the low reference limit at the late period of lactation. This tendency could be the effect of metabolic imbalance and selenium deficiency in sheep diet.