Whilst a considerable number of studies have been reported on the acute toxicity of nanoparticles (NPs) on invertebrates such as Daphnia magna, few studies have been reported on the biochemical change (biomarkers) induction on these species by NPs, especially metal oxide NPs. The aim of this study was to investigate some biomarkers in D. magna induced by copper oxide (CuO) and zinc oxide (ZnO) NPs under controlled laboratory conditions. We exposed the 5 day old D. magna for 72 h to sublethal concentration of CuO and ZnO NPs in synthetic moderately hard water (MHW) with and without dissolved natural organic matter (NOM) and estimated the glutathione-S-transferase (GST) activity, formation of oxidized glutathione (GSSG), and amounts of thiobarbituric acid reacting substances (TBARS) and metallothionein (MT). Additionally, complementary short term dissolution studies on CuO and ZnO NPs were conducted. The results showed inactivation of GST enzyme by both metal oxide NPs. The results also showed increased production of oxidized GSH, increased generation of TBARS and increased induction of MT. In the presence of NOM, significant reduction (p<0.05) in these biochemical changes was observed. These results indicated that oxidative stress is one of the toxicity mechanisms for these metal oxide NPs. Furthermore, the results suggest that these metal oxide NPs compromise the health of D. magna, and possibly other aquatic organisms, and therefore have potential to affect ecosystem stability. The short term dissolution studies showed that the proportion of dissolved NPs is higher (1.2% and 70% of initial concentration for dissolved Cu and Zn, respectively) at low particle concentration and is lower (0.4% and 17% of initial concentration for dissolved Cu and Zn, respectively) at higher particle concentration. These results suggest that the observed toxicity may be caused by both metal oxide nanoparticles and metal ions dissociated from the nanoparticles.