Recently gold nanoparticles (GNPs) have been actively studied as photothermal converters, drug carriers, and imaging agents in a wide range of applications in cancer diagnosis and therapy. The prolonged peroral administration of GNPs in a range of sizes was performed to investigate the morphological changes and their reversibility in the internal organs of laboratory animals. In this study, GNPs with average diameters of 2 nm, 15 nm and 50 nm were administered during 30 days, and the reversibility of morphological changes was investigated 14 days after administration. After the prolonged administration of GNPs, the severity of morphological changes in the liver, kidney, spleen and lymph nodes depended on the nanoparticle size. Specifically, 50 nm nanoparticles caused the most pronounced dystrophic and necrobiotic effects, whereas the smallest 2nm nanoparticles caused proliferative changes. Most importantly, the development of pathological processes was reversible, as evidenced by the gradual restoration of the organ structure at 14 days after the end of GNPs administration.