A comparative morphological and histochemical investigation has been performed on changes in the female rat adrenals, thymus and spleen under a 5 hours' immobilization and after its termination. During the first 24 h after the 5 hours' immobilization in the adrenals and in the lymphoid organs changes specific for an acute stress reaction develop. These changes reach their maximum at different time in the lymphoid organs and in the adrenals, and there is not any definite time correlation between morphological manifestations of the acute stress reaction--delipoidization and hypertrophy of the adrenals and the destructive changes in the target-organs (thymus, spleen). As a rule, the destructive changes in the thymus appear earlier than those of the widespread delipoidization and hypertrophy of the adrenals and are much earlier morphological signs of an acute stress reaction. As a whole, the stress changes in the female rat lymphoid organs develop more quickly than in the male rats. A rapid rise of the acid phosphatase activity in the spleen macrophagal elements under the stress is an early sign demonstrating the development of an early stressory reaction and can be used for its histochemical indication.