Basal level of 11-hydroxycorticosteroids in plasma was measured during one year period in spermophiles and correlated with a physiological state of the animals. Maximum level was found in Spring after the animals had awaken from hibernation, while a minimum level was observed during hibernation. During the waking up process the level of corticosteroids increased slowly which may show that this process does not act as a stressor. In contrast, the falling asleep process starts under a decreased level of corticosteroids in plasma and is accompanied by a further decrease. Maximum stimulatory effect of adrenaline and isoprenaline on pituitary-adrenal axis was found after waking up in Spring. It was considerably less in Summer, while no such effect was found before hibernation period. However, at the same time the sensitivity of adrenals to ACTH was sufficient. From this it may be concluded that seasonal changes in pituitary-adrenal response to catecholamines may be due to changes in central regulatory mechanisms.