Increasing use of nickel oxide (NiO) nanoparticles in different applications results in high occupational and environmental exposure to them. However, the effect of NiO nanoparticles on human health is still poorly documented. It was aimed to investigate the toxic potentials of NiO nanoparticles on NRK-52E kidney epithelial cells. The following assays were used: the nanoparticle characterization by transmission electron microscopy (TEM) and dynamic light scattering (DLS); the determination of cellular uptake and morphologic changes by TEM and inductively coupled plasma-mass spectrometry (ICP-MS); MTT and neutral red uptake (NRU) assays for cytotoxicity; comet assay for genotoxicity; the determination of malondialdehyde (MDA), 8-hydroxydeoxyguanosine (8-OHdG), protein carbonyl (PC) and glutathione (GSH) levels by enzyme-linked immune sorbent assays (ELISA) for the potential of oxidative damage; and Annexin V-FITC apoptosis detection assay with propidium iodide (PI) for apoptosis. The nanoparticles were taken up by the cells and induced dose-dependent DNA damage by comet assay and oxidative damage evidenced by increasing levels of MDA, 8-OHdG, PC and depletion of GSH. At >= 294.0 mu g/mL concentration, NiO nanoparticles caused 50% inhibition in cell viability by the cytotoxicity assays. Also, they showed apoptotic/necrotic effects on the cells as well as some morphological changes. We have indicated that their cellular damage effects should raise concern about the safety associated with their applications in consumer products.