The current dietary allowance for selenium (Se) for pigs does not consider Se requirements for expression of several newly discovered Se-dependent enzymes and has raised environmental concerns. Our objective was to determine dietary Se requirements of young pigs for the full expression of cellular (GPX1), plasma (GPX3) and phospholipid hydroperoxide (GPX4) glutathione peroxidases. In Experiment 1, 18 weanling male pigs (4 wk old) were fed a corn-soybean meal basal diet (BD, 0.03 mg Se/kg) with the addition of 0, 0.1 or 0.3 mg Se/kg (Na2SeO3). In Experiment 2, 24 weanling barrows (6 wk old) were fed a similar BD with the addition of 0, 0.2, 0.3 or 0.5 mg Se/kg. Both experiments lasted for 5 wk. Pigs fed the BD had lower (P < 0.05) tissue GPX1 and GPX4 activities, plasma GPX activity, and(or) plasma Se concentrations than those fed the Se-supplemented diets. In Experiment 1, GPX1 and GPX4 activities in liver, heart and lung were lower (P < 0.05) in pigs fed 0.1 mg Se/kg than in those fed 0.3 mg Se/kg, although no such differences existed in thyroid or pituitary. Pigs fed 0.1 mg Se/kg also had lower (P < 0.05) plasma GPX3 activity at wk 5 and higher (P < 0.05) hepatic glutathione S-transferase activity than pigs fed 0.3 mg Se/kg. In Experiment 2, GPX1 and GPX4 activities in liver and heart, GPX1 and GPX4 mRNA levels in liver and GPX3 activity in plasma exhibited plateaus at 0.2 mg Se/kg. Pigs fed the BD had greater concentrations of F2-isoprostanes (a marker of in vivo lipid peroxidation) than those fed 0.2 mg Se/kg in plasma (P < 0.03) and liver (P < 0.04). We conclude that supplemental Se at 0.2 mg Se/kg of diet is required to support the full expression of three Se-dependent glutathione peroxidases in young pigs.