The present study has evaluated effect of fluroxypyr concentrations 0-0.8 mg l(-1) (a widely-used herbicide for controlling annual or perennial weeds growth) on selected metabolic and stress-related parameters in Oryza sativa plants after 6 days of exposure. Increasing concentrations decreased shoot growth and accumulation of chlorophylls but had no effect on root biomass. Increasing doses led also to increase in superoxide radical, hydrogen peroxide and proline accumulation, while malondialdehyde, an indicator of lipid peroxidation, was constitutively elevated. Histochemical staining with nitroblue tetrazolium and 3, 3-diaminobenzidine were positively correlated with the generation of superoxide radical and H(2)O(2). The fluroxypyr-induced oxidative stress triggered significant changes in activities of superoxide dismutase, catalase, ascorbate peroxidase and peroxidase (POD). Activities of the antioxidant enzymes show a general increase at low fluroxypyr concentrations and a decrease at high fluroxypyr levels (except for POD). Analysis of naturing polyacrylamide gel electrophoresis confirmed these results. These data support the observation that fluroxypyr-triggered oxidative stress was responsible for the disturbance of the growth in the rice plants.