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Aim. Study of effects of titanium dioxide nanoparticles administered orally on the rodent thymus morphological and functional state. Methods. The study used classical morphological approaches and specific markers of proliferation - Ki-67, apoptosis - p53 protein and macrophages - CD68, which allow adequately identifying not only the cells themselves, but also their functional state. The rodent thymus was analyzed after oral administration (10 mg/kg of body weight of the animal, 28 days) of the TiO2 nanoparticles (rutile form, 40-60 nm) obtained by diluting the TiO2 powder in distilled water. Aggregation of the nanoparticles was prevented by processing a suspension of nanodispersed TiO2 in an ultrasonic bath. The rats of the control group were orally injected with distilled water in the same volume. Serial paraffin sections of the thymus were stained with hematoxylin-eosin, picrofuxin according to Van Gieson, immunohistochemical staining was performed with antibodies to Ki-67, CD68, p53. Results. Significant changes of the tissue of thymus gland under the influence of TiO2 nanoparticles were revealed. In the thymus of experimental animals, a decrease in the proportion of cortex was found to be 17.6 %, a significant decrease in the density of the cell population due to decrease in the number of thymocytes was demonstrated. Immunohistochemical typing revealed that under the influence of TiO2 nanoparticles, a decrease in the number of Ki-67-positive cells in the cortex of the lobule of the thymus gland was observed, which indicates the inhibition of proliferation in these conditions. Under the influence of TiO2 nanoparticles, an increase of the amount of cells entering apoptosis in the cortex of the thymus segment of the experimental group by 5.18 times was observed, as evidenced by the results of immunohistochemical study of the expression of apoptosis marker p53 protein. Perhaps, as a compensatory mechanism there is a pronounced increase in the number of macrophages, as indicated by an increase in the average number of CD68 immunopositive cells in the cortex of the thymus of the test group by 2.61, and in the brain substance by 1.35. Conclusion. The revealed morphological and functional changes of the thymus with oral administration of TiO2 nanoparticles indicate their immunosuppressive effect.