The aim of the investigation was to study the effect of upconversion nanoparticles (UCNPs) with different surface coatings on viability and functional neural network activity of primary hippocampal cultures. Materials and Methods. The UCNPs with three surface coating modifications (tetramethylammonium hydroxide (TMAH), polymaleic acid octadecene and polyethyleneimine (PEI)) were synthesized by a solvothermal technique with further hydrophilization. Primary hippocampal cultures, obtained from C57BL/6 mice embryos (Е18), were incubated with tested UCNPs in the concentration of 0.8 and 8 mg/ml during 72 h. The cell viability detection, evaluation of morphological changes by immunocytochemical staining as well as the UCNPs influence on the main parameters of the functional neural network calcium activity and on the endocytosis processes were carried out. Results. Our studies revealed that UCNPs caused a dose-dependent cytotoxic effect on primary hippocampal cultures, wherein the severity of this effect directly related to the type of UCNPs surface coating. The greatest cytotoxicity was identified for UCNP–PEI, and the least — for UCNP–TMAH. UCNPs toxicity is manifested in significant morphological changes of neural networks and in the increase the number of dead cells (р