In experiments on the Arctic ground squirrel Citellus parryi, radioimmune assay of the content of thyrotropin of the hypophysis and thyroxine and triiodothyronine of the thyroid in the peripheral blood has been made at monthly intervals from July until May. It was found that during hibernation period, thyroxine and triiodothyronine concentrations in the blood of sleeping animals are high as compared with those during pre-hibernation period in autumn and active period in May. Thyrotropin content of the blood increases from October to May, being the lowest however during the season of the deepest hibernation from December to February. It is suggested that activation of the hypophysial--thyroid system after resting period in summer and autumn begins in October. During deep sleeping, it is depressed, being recovered in March. High levels of the thyroid hormones during hibernation period may be explained by metabolic strategy of the organism in hibernating animals which is directed to optimization of energy supply of hibernation.