Boron neutron capture therapy is considered a promising method for the treatment of malignant tumors of the head and neck. It is believed that to increase the effectiveness of this type of therapy, the use of large doses of boron is required, which may entail damaging effects on healthy tissue. One of the substances used in the clinical practice of boron neutron capture therapy is sodium boroncaptate Na2B12H11SH (BSH), enriched with the 10B boron isotope. The purpose of the study was to study the structural reactions of the myocardium and liver of CD-1 mice after administration of BSH. A light-optical and polarization-microscopic study of the myocardium and liver of male CD-1 mice (n=56) was carried out after injection of a boron-containing substance in doses of 100 and 1000 mg/kg, once, intraperitoneally. Assessment of structural changes in the myocardium and liver was carried out 1, 3 and 7 days after BSH administration. A single injection of BSH at a dose of 100 mg/kg did not lead to the death of animals, whereas 3 hours after the injection of BSH at a dose of 1000 mg/kg, 1 animal died. The body weight of the animals changed slightly during the experiment. Analysis of heart weight showed a decrease in this indicator on days 3 and 7 compared with indicators in the same groups on day 1 of the experiment. When analyzing the dynamics of changes in liver mass, no significant changes were revealed during the experiment. The main structural changes in the myocardium included lytic and contractural damage to cardiomyocytes, hemodynamic disturbances in the form of pronounced venous and capillary congestion. Liver damage was manifested in dystrophic changes in hepatocytes, the appearance after 3 days of monocellular necrosis of hepatocytes and pericentral mononuclear infiltrates. The data obtained indicate that the used doses of BSH 100 and 1000 mg/kg with a single injection cause structural changes in the myocardium and liver of varying severity, which persist for 7 days of observation.