The use of soil additives such as selenium can positively influence the antioxidative system of plants, making them more tolerant to abiotic stresses. The aim of this work was to evaluate the concentration of sodium selenite or sodium selenate applied to the substrate that causes improve in plant growth and antioxidative enzyme activities in sunflower plants. The treatments were divided in: control (absence of selenium); 0.2 mg L-1 of sodium selenate; 0.4 mg L-1 of sodium selenate; 0.8 mg L-1 of sodium selenate; 0.2 mg L-1 of sodium selenite; 0.4 mg L-1 of sodium selenite and 0.8 mg L-1 of sodium selenite. The analysis of Shoot Dry Mass (SDM) production and activities of the antioxidantive enzymes: Ascorbate Peroxidase (APX), Guaiacol Peroxidase (GPX) and Catalase (CAT) was performed. For SDM and APX the concentration of 0.8 mg L-1 of sodium selenite caused higher values. CAT showed greater activity in treatments that received 0.4 and 0.8 mg L-1 of sodium selenate and 0.4 and 0.8 mg L-1 of sodium selenite than the control treatment. GPX showed superior activity in the treatments 0.8 mg L-1 of sodium selenate, 0.2 mg L-1 of sodium selenite and 0.8 mg L-1 of sodium selenite than the control treatment. It was concluded that selenium promoted improvements in the antioxidant activity and in the production of shoot dry mass of sunflower plants.