The effects of feed supplementation with inorganic and organic forms of selenium (Se) on the activities of blood glutathione peroxidase (GPx), superoxide dismutase (SOD) in erythrocytes as well as on total antioxidant status in plasma (TAS) were examined in lambs. Fifteen animals were divided into three groups and fed experimental diets for 3 months. The first group received the basic diet (BD) providing a daily intake 50.6 μg of Se only. Diets for the second and third groups consisted of BD supplemented with selenium 0.3 mg ∙ kg ^-1 of DM in the form of sodium selenite or Se-enriched yeast and giving a total daily intake 278.6 μg of Se per animal. Increases in Se concentrations in whole blood, plasma and red blood cells as well as in the activities of blood GPx were highly significant in both supplemented groups, but no differences could be found that were due to the form of Se administered. The activity of SOD in erythrocytes was found to be significantly higher in the selenite group of lambs compared with both groups given BD or Se-yeast. The total antioxidant status was significantly lower in the group given Se-yeast compared with animals on BD, which might be explained by larger amounts of selenide continuously produced from body Se deposits. The presented results support the opinion on the pro-oxidative features of sodium selenite when it is utilized as an animal feedstuff additive.