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The toxic effects of CdSe/ZnS QDs on zebrafish (Danio rerio) embryos at different developmental stages were investigated in this study. The voluntary movement frequency, body length, hatching rate, mortality and malformation rate, SOD activities, MDA contents, mRNA expression of metallothionein (MT) and heat stress protein 70 (Hsp70) were used as indicators. The results showed that the EC50 was 316.994 nmol x L(-1) for zebrafish embryos (72 hpf) when exposed to CdSe/ZnS QDs. After the CdSe/ZnS QDs exposure, the embryos showed a significant increase in mortality and malformation rate, a decrease in hatching rate and body length, an advance in hatching time, and a changing in the spontaneous movement frequency, and many other toxic effects, such as the condensation of embryonic eggs, the formation of pericardial cysts and curvature of the spine. Moreover, it was found that the MDA contents in the embryos in CdSe/ZnS QDs groups were significantly increased, and the SOD activities were changed. In addition, the mRNA expression level of MT and Hsp70 were up-regulated. All the information suggests that exposure of CdSe/ZnS QDs can cause toxic effects on zebrafish embryos, and the effects may be related to the releasing of Cd2+, particle size and oxidative stress.