Arsenic (As) is toxic to a wide range of organisms including plants. Excessive application of arsenical pesticides in agricultural lands has resulted in elevated levels of As in soils. Studies on arsenate, which is the predominant form of As found in aerobic soils, has shown that plant species not resistant to As suffer considerable stress upon exposure. Phytotoxicity of As occurs due to its ability to enter biochemical reactions in the place of phosphate. The major symptoms of As stress include growth inhibition, chlorosis, defoliation and water-deficiency stress. The objective of this study was to gain insight into the biochemical stress response mechanism in rice seedlings treated with arsenical pesticides in a sandy soil (Immokalee series) under greenhouse conditions. The soils were amended with sodium arsenate and dimethylarsenic acid (DMA) at two rates: 675 and 1500 mg/kg As. Rice seedlings were used to study plant response to organic and inorganic As in the pesticide-amended Immokalee soil. Rice plants were grown in columns for six months in a temperature and humidity controlled greenhouse. Shoot tissues were collected at the end of six months to study antioxidant responses. Results show increased activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and Glutathione peroxidase (GPx) in rice seedlings treated with As. The importance of antioxidant enzymes in protecting rice seedlings from arsenic stress is discussed.