In the blood of 56 European bison, the levels of T4, T3, and the ratio of T3:T4 and cortisol were studied. 2. Between December and March, the T4 level changed from about 53 to 83 ng/ml of blood serum but in April it increased to 90 ng/ml. 3. The level of T3 in this period was 0.817-1.475 ng/ml and in April it was 2.40-3.40 ng/ml. 4. The ratio of T3:T4 was 0.015-0.024 and in April it increased to 0.027-0.052. 5. The level of cortisol changed from 1.00 to 6.70 ng/ml and in April it was from 1.00 to 14.00 ng/ml.