Oxidative stress and ultrastructural changes under hexavalent chromium stress were investigated in developing rice seedlings. Chromium treatment for 24 or 48h resulted in inhibition of root length and dry biomass. Atomic absorption spectrometry analysis of roots showed that chromium accumulation increased with increase in concentration and duration of metal treatment. Chromium resulted in increased production of hydrogen peroxide and superoxide radical in root cells, which was a significant change after 48h of treatment. Time-course analysis of malondialdehyde content showed no substantial variation during early treatment periods (2, 6 or 12h). Increase in malondialdehyde content was observed only after 18h and it continued to increase until 48h after treatment. Loss of membrane integrity, analyzed in terms of Evans blue uptake in root cells, showed an increase in uptake of the reagent, indicating loss of membrane integrity. The antioxidant enzyme, viz., guaiacol peroxidase, was least affected, while glutathione reductase showed significant decline after 24 or 48h of metal treatment, followed by increased activity of superoxide dismutase. The level of ascorbate was not affected by chromium, while an increase in the level of glutathione was observed. At the ultrastructural level, potential damage to the root cell was noted after 48h at 100microM of chromium compared with controls.