Selenium concentration and GSH-Px activity in blood serum of the boars control and experimental group were similar. Other authors have not found a clear relationship between the quantity of this microelement in feeding ration and its concentration as well as GSH-Px activity in porcine blood serum. MARIN-GUZMAN et al. (1997) found statistically significant increase in selenium content and GSH-Px activity in blood serum of boars obtaining 0.5 ppm Se in ration during growing. MAHAN and PARRETT (1996), on the other hand, who applied 0.1, 0.3 and 0.5 ppm Se in the form of sodium selenite in the ration found very similar selenium concentration and GSH-Px activity in the blood serum of growing and adult pigs.