Poster (download) Shurlygina A.V1, Serykh A.E.2, Rachkovskaya L.N.3, Michurina S.V.4, Rachkovsky E.E.5, Letyagin A.Yu.61Research Institute of Clinical and Experimental Lymphology – Branch of the Institute of Cytology and Genetics SB RAS., anna_v_s@mail.ru2Research Institute of Clinical and Experimental Lymphology – Branch of the Institute of Cytology and Genetics SB RAS, .Novosibirsk State University, rasiel1996@yandex.ru3Research Institute of Clinical and Experimental Lymphology – Branch of the Institute of Cytology and Genetics SB RAS, noolit@niikel.ru4Research Institute of Clinical and Experimental Lymphology – Branch of the Institute of Cytology and Genetics SB RAS, s.michurina@ngs.ru5Research Institute of Clinical and Experimental Lymphology – Branch of the Institute of Cytology and Genetics SB RAS, noolit@niikel.ru6Research Institute of Clinical and Experimental Lymphology – Branch of the Institute of Cytology and Genetics SB RAS, letyagin-andrey@yandex.ru In modern time person’s everyday life, the light regime is often disrupted. It entails a decrease in the synthesis of melatonin. Melatonin influences the differentiation of lymphocytes, the activity of inflammation, and the strength of the immune response. Study\’s purpose is the identification of histological features of the thymus and spleen of mice by means of oral administration of a new melatonin-containing preparation (Complex M) in conditions of light regime\’s disruption under round-the-clock illumination. In the experiment, we used mice with SPF status (inbred strain C57Bl / 6J), males, aged 10-12 weeks. It was shown that the introduction of Complex M reduces the relative size of spleen\’s white pulp, increases the size of the reactive centres, the marginal zone of the spleen follicles and venous vessels of the organ. In addition, the introduction of Complex M increases the cortical-brain index of the thymus, which may indicate This may be a sign of activation of immune functions, increased venous blood flow and stimulation of the central differentiation of T-lymphocytes.