In modern-day conditions, the prevalence of situations in which light disrupts one's daily life has increased. This causes a decrease in the synthesis of melatonin at nighttime. Melatonin affects the body's lymphocytes differentiation, inflammation and immune responses. Therefore it is important to study the effect of light disturbance on the organs of the immune system as well as the possibilities of correcting immunity disorders with the help of a new melatonin-containing drug (Complex M), synthesized in the laboratory of pharmaceutical technologies of Research Institute of Clinical and Experimental Lymphology (RICEL), a branch of the Institute of Cytology and Genetics (ICG). The purpose of this study is to identify the morphological features of the thymus and spleen of mice after intragastric administration of the Complex M within 7 days alongside round-the-clock illumination. The experiment used male C57Bl/6J SPF mice aged 10-12 weeks. The day after the last administration of Complex M, the animals were sacrificed by cranio-cervical dislocation and the thymus and spleen samples were taken. The spleen and thymus samples were fixed by immersion in 10% buffered formalin and enclosed in Histomix. Sections were stained with hematoxylin and eosin and morphometric analysis was performed using ImagePro and Image J programms. The administration of Complex M led to an increase in the diameter of the pulp veins and an increase in the cortico-medullary index of the thymus, which indicates its activating effect on the central differentiation of T-lymphocytes.