AbstractThere is no accurate information on the effect of metal oxide nanoparticles (MO NPs) on plant enzymatic activities (EAs) under drought stress (DS). The aim of this work was to study the effect of MO NPs on the EAs of Agropyron cristatum (L.) Gaertn and Sanguisorba officinalis L. under DS. Foliar spray of silver oxide (AgO), iron oxide (FeO), zinc oxide (ZnO) and cadmium oxide (CdO) at levels of 30, 60, 90 and 120 mg L−1 were used on plant aerial parts under DS levels of 25–100% field capacity (FC). Glutathione reductase (GR), catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) were measured after 2 weeks of the experiment. The EAs changes varied depending on the plant species, levels of DS, types and levels of NPs. Application of 30–60 mg L−1 MO NPs under 25–50% FC significantly reduced EAs, especially in S. officinalis (P < 0.01). Higher concentrations of MO NPs significantly increased EAs. Decreases in CAT (20.90%, 18.80%), POD (21.30%, 17.67%) and SOD (23.14%, 16.88%) were observed using 60 mg L−1 ZnO NPs under 25–50% FC in S. officinalis and A. cristatum, respectively. High concentrations of CdO NPs (90–120 mg L−1) increased EAs (max. = 24.0%) in both species under 25% FC.