The effects of arsenite treatment on generation of reactive oxygen species, induction of oxidative stress, response of antioxidative system, and synthesis of phytochelatins were investigated in two indica rice (Oryza sativa L.) cvs. Malviya-36 and Pant-12 grown in sand cultures for a period of 5-20 days. Arsenite (As(2)O(3); 25 and 50 μM) treatment resulted in increased formation of superoxide anion (O (2) (.-) ), elevated levels of H(2)O(2) and thiobarbituric acid reactive substances, showing enhanced lipid peroxidation. An enhanced level of ascorbate (AA) and glutathione (GSH) was observed irrespective of the variation in the level of dehydroascorbate (DHA) and oxidized glutathione (GSSG) which in turn influenced redox ratios AA/DHA and GSH/GSSG. With progressive arsenite treatment, synthesis of total acid soluble thiols and phytochelatins (PC) increased in the seedlings. Among antioxidative enzymes, the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), total ascorbate peroxidase (APX, EC 1.11.1.11), chloroplastic ascorbate peroxidase, guaiacol peroxidase (EC 1.11.1.7), monodehydroascorbate reductase (EC 1.6.5.4), and glutathione reductase (EC 1.6.4.2) increased in arsenite treated seedlings, while dehyroascorbate reductase (EC 1.8.5.1) activity declined initially during 5-10 days and increased thereafter. Results suggest that arsenite treatment causes oxidative stress in rice seedlings, increases the levels of many enzymatic and non-enzymatic antioxidants, and induces synthesis of thiols and PCs, which may serve as important components in mitigating arsenite-induced oxidative damage.