The experiments on white rats have shown that the induction of 4 hour stress produces an acute increase in beta-endorphin level, as well as characteristic changes in ACTH, cortisol, insulin, thyroxin and triiodothyronine concentrations. Different types of adaptation (training with short stress periods or injection of rhodiola rosea extract) promote a moderate increase in the amount of serum immunoreactive beta-endorphin, preventing its subsequent stress-induced elevation. Adaptation is characterized by a decrease or total prevention of hormonal changes peculiar to stress. The role of opioid neuropeptides in enhancing stress tolerance and the effect of adaptation factors are discussed.