Upconversion nanoparticles (UCNPs) are of interest as novel luminescent probes for numerous applications in nanobiophotonics. The small size of UCNPs enables the particles to overcome biological barriers, thereby ensuring a deep penetration into the tissues and accumulation in a number of organs. In addition, particles are known to possess high surface chemical reactivity as well as a large surface-tovolume ratio, which affect their biocompatibility. In this paper, we present data of the dark toxicity of uncoated UCNPs in the course of their interaction with tissues and organs. It was found inflammatory reaction that developing in the organism when UCNPs are introduced under the skin. The signs of damage and necrosis in all layers of the skin were not observed. The dense connective tissue capsule was formed around the particles through a week after their administration. The particles do not diffuse and dissolve, but remain at the introduction site. UCNPs cause an inflammatory reaction in skin, in a week the skin site, where particles were administrated, is almost not seen. The changes in blood supply and blood circulation in the organs were developed in 2 days mainly. This in turn led to insignificant dystrophic changes. All changes were reversible and disappear or their degree of severity decreases after 7 days after the administration of the particles. The particles enter the total blood flow, but they did not show a pronounced toxicity.