It has been suggested that the preventive administration of cryopreserved cord blood nucleated cells (CBNCs) prior to the onset of desynchronosis may help to correct its negative effects on the body. The efficacy of CBNCs on blood leukocyte indicators in young and old rats (6 and 18 months old) was investigated. Desynchronosis was modelled by shifting the light regime: the duration of the light period was increased by 12 h, resulting in a light period of 24 h. Leukocyte types were determined in blood smears. Integral leukocyte indices were used to assess the state of the immune system. Desynchronosis caused leukocytosis in young rats and leukopenia in aged rats. The number of banded neutrophils increased in both, segmented neutrophils decreased in young rats and increased in aged rats; lymphocytes changed on the contrary; eosinophils decreased in young rats but did not change in aged rats. In young rats with desynchronosis, the predominance of young cells, macrophages, activation of the humoral link of the immune system, auto- and endogenous intoxication, acceleration of hypersensitivity of the immediate type processes, and increased body adaptation were noted. In aged rats, infectious intoxication, cellular link of immune system predominance, and a decrease in allergy and adaptation were observed. After CBNCs injection before desynchronosis, leukocytosis remained, segmented neutrophils and eosinophils recovered, banded neutrophils increased and lymphocytes decreased in young rats. In old rats, the total number of leukocytes, monocytes and eosinophils recovered, but lymphopenia increased. Regardless of age, the cells of non-specific protection predominated; the cellular link of the immune system activated, adaptation decreased and autointoxication was manifested. In young rats, young forms of neutrophils increased, allergy and immunoreactivity decreased, and the affective link of the immune system and macrophage in aged rats was activated. Therefore, after the preventive use of human CBNCs before desynchronosis initiation in aged rats, the total leukocyte content and the number of eosinophils and monocytes were restored. In 6-month-old rats, the content of young forms of neutrophils increased, indicating stimulation of leukopoiesis. The allergy index decreased in both groups.