Arsenic environmental contamination and the toxic effects it causes on living organisms is a worldwide issue. Most Southeast Asian countries are dependent on As-contaminated groundwater for drinking and irrigation purposes, leading the latter to the contamination of agricultural soils as well. Rice (Oryza sativa L.), which is strongly susceptible to accumulate As in its grains, is a staple food for more than half of the world’s population, and many of the rice producing countries suffer from As contamination in their groundwater or soil. Arsenic can be found in both inorganic (arsenite (As(III)) and arsenate (As(V))) and organic (methylated) forms in anaerobic and aerobic soil/water environments. Arsenic uptake by plants is mainly as As(V), but, in flooded paddy soils As(III), which is assumed to be more toxic than As(V), is normally the predominant species. Therefore, the culture of rice under oxidizing aerobic conditions has been recommended in fields suffering from As contamination. Chemical analogy with phosphate and binding to thiol-rich peptides, gives As(V) and As(III), respectively, its potential to be toxic. Arsenate toxicity, as well as other heavy metals or environmental stresses, has been proved to stimulate reactive oxygen species (ROS) formation and lead to oxidative stress. But there is still not enough information about the relationships between As accumulation and plant oxidative stress. A hydroponics study was carried out to evaluate the effects of increasing As concentrations on oxidative stress parameters in rice plants grown in optimized hydroponic conditions. The aim was to obtain prediction equations which may help to foresee rice behavior and tolerance and phytotoxicity thresholds. A preliminary hydroponic experiment was carried out in order to stablish the most favorable conditions for plant growth, using a fixed arsenate dose (1 mg L^-1) and different pH and DOC concentrations were tested. Then, in the main experiment, rice plants were exposed to increasing As doses (0-10 mg L^-1) at pH 6.0 and 100 mg L^-1 DOC. Rice plants were harvested after 21 days of treatment exposure. Trace element and nutrient concentrations in the plants were determined (ICP-OES) after microwave assisted acid digestion. Arsenic major species were analyzed using HPLC-AFS in PBS sample extracts. Rice leaves were also assessed for oxidative stress through lipid peroxidation (MDA) and protein oxidation (carbonyl proteins) determinations. Plant yield (shoot and root length and dry weights) decreased strongly with As concentrations above 2.5 mg L^-1. Arsenic concentrations increased with As dose in plant shoots and roots, showing higher accumulation in the roots, except in the highest doses (5 and 10 mg L^-1) where As concentrations in below and above parts were similar. Moreover, As(V) accumulation was higher in the aerial parts and As(III) in the roots. Lipid peroxidation (MDA concentration) increased with As dose and appeared to be related to As(V) concentrations in the roots. Carbonylated proteins increased also with As dose, but were related to Cu and As(III) concentrations in the aerial part of the plants. These results may help to establish As toxicity thresholds and will be of interest for future studies regarding As accumulation and oxidative stress/toxic effects in rice plants.