Effect of experimental hypothyreosis, caused by various thyrostatic agents, on content of sialic acids was studied in blood plasma and liver tissue of white rat males. In hypothyreosis content of sialic acids was distinctly increased in blood serum but decreased in homogenate and structure elements of liver tissue. Alteration in content of sialic in blood plasma correlated with their content in liver structure elements. Content of sialic acids in blood plasma and liver tissue was shown to depend on dosage and length of administration of various thyrostatic preparations. Mechanism of increase in concentration of sialic acids in blood serum under influence of experimental hypothyreosis is discussed.