Adverse effect of engineered nanoparticles (NPs) on the aquatic environment and organisms has recently drawn much attention. This paper reports on the toxicity of CuO NPs to juvenile carp (Cyprinus carpio) and their distribution in the fish. CuO NPs and its counterpart bulk particles (BPS) (10, 50, 100, 200, 300, 500 and 1000 mg L(-1)) exhibited no acute toxicity (96 h), while during the 30 day sub-acute toxicity test, carp growth was significantly inhibited by CuO NPs (100 mg L(-1)) in comparison to control, CuO BPs and Cu(2+) groups. CuO NPs (or released Cu(2+) ions inside the fish body) could distribute in various tissues/organs and followed an order: intestine > gill > muscle > skin and scale > liver > brain. For time-related distribution. Cu content (expressed on a dry mass basis) in intestine, gill and liver increased faster (within 1 day) and they had obviously higher Cu content than other tissues/organs at all exposure times. CuO NPs could be excreted by carp to lower their toxicity. Cholinesterase activity was inhibited during CuO NPs exposure, suggesting NPs exposure could have potential neurotoxicity, and free Cu(2+) ions dissolved inside the carp body was responsible for the cholinesterase inhibition. Finally, actual suspended NPs concentrations should be used instead of initially added concentrations whenever possible in nanotoxicity studies. (C) 2011 Elsevier B.V. All rights reserved.