Аннотация:Studies of the effects of toxicants of different nature on the organisms at the ultrastructural level make it possible to find out the subtle mechanisms of the occurrence of certain damage to biological systems. The liver is a suitable object for such study because it is of interest as a normal model with weakly proliferating cells that are able to accumulate latent damage. Radiation-induced changes in living systems motivate scientists to research because of their uniqueness: γ-radiation has a wide spectrum of action, exerting various and, sometimes, ambiguous, effects on cells and organisms in general. The purpose of the study was to identify changes in rat liver cells after exposure to ionizing radiation at a dose of 5 Gy using light and electron microscopy methods, as well as measuring and processing some morphometric parameters (area of hepatocytes and their nuclei, mitochondria, number of hepatocytes and binuclear cells per unit area). The results of the experiment indicate the presence of significant transformations in the structure of the liver parenchyma: a violation of the typical structure of the plates of hepatocytes, a strong narrowing of the hepatic sinusoids, cell deformation. At the ultrathin level, fragmentation and redistribution of chromatin, and EPR dystrophy are recorded. On the 40th day after γ-irradiation, the activation of protective processes in the liver of rats was observed at the cellular and subcellular levels – an increase in the proportion of binuclear and dark hepatocytes, hyperplasia of peroxisomes, and induration of the mitochondrial matrix. The enhancement of proliferative activity in the liver of rats exposed to a single ionizing radiation dose of 5 Gy was not confirmed on the 40th day after irradiation.