Age-related peculiarities of the adaptation of the hypothalamo-hypophyseo-adrenal system (HHAS) to the effect of heterotypic stress was studied experimentally in the context of stress-associated behavioral reactions. Young (3 month old), mature (6 month old) and aging (12 month old) Sprague Dawley rats (total number of animals equal to 36) were exposed to chronic heterotypic stressors for 7 days with the subsequent testing of their behavioral responses. Histological changes were studied in the hypothalamus, pituitary and adrenal glands as compared to age-matched control; immunohistochemical reactions were performed to demonstrate CRF, ACTH, ED1, PCNA and caspase-3 with subsequent image analysis. In the aging organism, as compared to young and mature animals, the degree of HHAS activation indicated the dissociation in its central part and adaptive desensitization prevention, typical to young and mature animals. Specifically, in the aging animals exposed to stress, high hypothalamic CRF expression was noted in association with relatively low hypophyseal ACTH expression and high level of adrenal activity. Reduced HHAS plasticity in the aging animals as compared to that in the other age groups, corresponded to their behavioral changes, demonstrating the reduced capacity of the aging organism to adapt to the exposure of unpredictably changing stressors.