Analysis of blood flowing from the rat thyroid has shown that an acute immobilization stress enhanced thyroid hormone secretion with an increase of the T3/T4-100 index. The most marked response to stress was observed after repeated 2 min immobilization with a 3 min interval: Such increase of secretion was arrested by injection of prasosin, an -adrenoblocker. Propylthiouracil injected an hour before the experiment reduced T4 thyroid conversion index. These results permit a conclusion that short-term repeated immobilization enhanced thyroid hormone secretion via -adrenergic system stimulation, this being paralleled by increased production of hormonally more active T3.