An experiment was designed to study the effects of a selenium (Se) supplementation with sodium selenite on the blood activity of glutathione peroxidase (GSH-Px; EC 1.11.1.9) and live weight gain of selenium deficient grazing dairy heifers from two dairy farms from the south of Chile. A total of 80 Friesian heifers (40 growing heifers, 287 kg b:w; and 40 mating heifers, 358 kg b:w) managed at grazing on permanent pastures belonging to two selenium deficient herds (40 animals from each herd), were used. Animals were allotted in similar groups, treated and control. The animals from the treated groups were administered with sodium selenite at 1.67% i.m., in a single dose of 5 mg of Se/100 kg body weight. Body weight change and GSH-Px blood activity were determined previous to supplementation and every three months. The initial blood activity of GSH-Px was 34 U/g Hb in one herd and 116 U/g Hb...