Changes in the sum total activity of cholinesterase of the plasma and erythrocytes (AC substrate) and also separately of acetyl- and pseudocholinesterase (MeC and ByC substrates) were studied in experiments on rats with thyrotoxin toxicosis and 6-methylthiouracil hypothyroidism. The sum total activity of cholinesterase proved to increase in the erythrocytes of rats with thyrotoxicosis and fell in the erythrocytes and plasma of animals with hypothyroidism. The activity of acetylcholinesterase of erythrocytes and plasma increased in thyrotoxicosis and decreased in hypothyroidism. The activity of pseudocholinesterase of erythrocytes and plasma decreased in thyrotoxicosis and showed no significant difference in hypothyroidism.