A radioimmunoassay showed that the stay of intact animals in the highlands (the pass of Tuya-Ashu, Kyrhyzstan republic, 3200 m) was accompanied by phase changes in the blood content of the thyroid hormones and thyrotropin (TSH). An increase in the T., concentration in the blood of healthy animals on the first days of adaptation was determined by thyroid activation. Further on thyroid activity suppression was observed in parallel with a decrease in the production of the thyroid hormones. The thyroid of animals with thyrotoxicosis reacted less to a stress situation (climbing the mountain) than that of intact animals. Experimental animals' stay in the mountains for a month resulted in a significant decrease in the content of the thyroid hormones and modification of signs of thyrotoxicosis. The production by the pituitary gland of TSH as a central regulator of thyroid activity was established at a higher level than that of the lowlands resulting in the suppression of hormono-poiesis. The results of comparative investigations were indicative of the adaptive-compensatory importance of fast elevation and normalization of thyroid hormonal activity to eliminate symptoms of thyrotoxicosis under the highlands hypoxic conditions.