The activity of plasma membrane (PM) nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and its catalytic properties in rice was investigated under drought stress conditions. Drought stress led to decreased leaf relative water content (RWC) and, as a result of drought-induced oxidative stress, the activities of antioxidant enzymes increased significantly. More interestingly, the intensity of applied water stress was correlated with increased production of H2O2 and O2- and elevated activity of PM NADPH oxidase, a key enzyme of reactive oxygen species generation in plants. Histochemical analyses also revealed increased H2O2 and O2- production in drought-stressed leaves. Application of diphenylene iodonium (DPI), an inhibitor of PM NADPH oxidase, did not alleviate drought-induced production of H2O2 and O2-. Catalysis experiments indicated that the rice PM NADPH oxidase was partially flavin-dependent. The pH and temperature optima for this enzyme were 9.8 and 40 °C, respectively. In addition, drought stress enhanced the activity under alkaline pH and high temperature conditions. These results suggest that a complex regulatory mechanism, associated with the NADPH oxidase-H2O2 system, is involved in the response of rice to drought stress. The activity of plasma membrane (PM) nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and its catalytic properties in rice was investigated under drought stress conditions. Drought stress led to decreased leaf relative water content (RWC) and, as a result of drought-induced oxidative stress, the activities of antioxidant enzymes increased significantly. More Intensity, the intensity of applied water stress was correlated with increased production of H2O2 and O2- and elevated activity of PM NADPH oxidase, a key enzyme of reactive oxygen species generation in plants. Histochemical Analyses also revealed increased H2O2 and O2- production in drought-stressed leaves. Application of diphenylene iodonium (DPI), an inhibitor of PM NADPH oxidase, did not alleviate drought-induced production of H2O2 and O2-. Catalysis experiments indicated that the rice PM NADPH oxidase was partially flavin-dependent. The pH and temperature optima for this enzyme were 9.8 and 40 ° C, respectively. In addition, drought stress enhanced the activity under alkaline pH and high temperature conditions. These results suggest that a complex regulatory mechanism, associated with the NADPH oxidase-H2O2 system, is involved in the response of rice to drought stress.