In order to improve the photocatalytic efficiency of titanium dioxide, doping it with metals has been suggested; however, toxicity studies of metal-doped nanoparticles on aquatic organisms are not completely known. Therefore, the purpose of this study was to evaluate the short term effect of exposure to Cr and Ba doped TiO2 nanoparticles (NPs) using gill histopathology of zebrafish (Danio rerio) under laboratory condition. In this study, four concentration levels, 1, 10, 32, and 100 mg L-1 of each Cr and Ba doped TiO2 NPs as well as a control group were used. After 4 days of exposure, from each experimental group, three samples of fish gill prepared to study the classic histology (haematoxylin – eosin). The results of this study showed that the Cr and Ba doped TiO2 NPs can cause gill alterations including aneurism, dilated and clubbed tips, hyperplasia, oedema, curvature, fusion of lamellae, increase of mucous secretion, and proliferation in the erythrocytes of cartilaginous core. The extent of tissue damages increased by concentration levels of NPs; however, the severity of injouries caused by Cr was more notable than that of observed by Ba.
