To study the biological effects of nanoscale copper oxide (nCuO), zinc oxide (nZnO), cerium dioxide (nCeO2) and their mixtures on Carassius auratus. Juvenile fish (Carassius auratus) were exposed to aqueous suspensions of nCuO, nZnO, and nCeO2 (alone and in mixtures) at concentrations of 20, 40, 80, 160, and 320 mg/L. The biomarkers-acetylcholinesterase (AChE) in brain, sodium/potassium-activated ATPase (Na+/K+-ATPase) in gill, and superoxide dismutase (SOD) and catalase (CAT) in liver-were determined after 4 days of exposure. Integrated biomarker response (IBR) was calculated by combining multiple biomarkers into a single value. AChE and SOD activities were significantly inhibited by all test metal oxide nanoparticles (NPs) at high concentrations (⋝160 mg/L) with the exception of nCeO2. Na+/K+-ATPase induction exhibited bell-shaped concentration-response curves. CAT activity was significantly inhibited at concentrations equal to or higher than 160 mg/L. The order of IBR values was nCeO2 ≈ nZnO/nCeO2 ≈ nCuO/nCeO2 < nCuO/nZnO/nCeO2 < nZnO < nCuO < nCuO/nZnO. The joint effect seemed to be synergistic for nCuO/nZnO mixtures, additive for the ternary mixture and less than additive or antagonistic for the binary mixtures containing nCeO2. Concentration-dependent changes of enzymatic activities (AChE, Na+/K+-ATPase, SOD, and CAT) were observed in fish exposed to nanoscale metal oxides. IBR analysis allowed good discrimination between the different exposures and might be a useful tool for the quantification of integrated negative effects induced by NPs toward fish.