['The present study was designed to determine the influence of a 12-week exercise program on the activity of erythrocyte glutathione peroxidase (GPX) in adolescents with Down syndrome.', 'An interventional study with before-after comparison.', 'Sport Medicine School, University of Cadiz (Andalusia, Spain).', 'Thirty-one male adolescents (16.3+/-1.1) with Down syndrome. None of them suffered acute medical problems at that moment and had not taken part in any physical activity program in the last 6 months.', 'A 12-week training program with 3 days per week, consisting of warm up (15 min) followed by a main part (20 to 35 min) at a work intensity of 60% to 75% of peak heart rate (HRmax=194.5-[0.56xage]) and by a cool-down period (10 min).', 'Erythrocyte activity of GPX.', 'Preexercise and postexercise GPX activity in adolescents with Down syndrome were 24.8+/-3.1 [23.1 to 26.5] U/g hemoglobin and 29.3+/-2.9 [28.1 to 30.5] U/g hemoglobin, respectively. When compared with baseline values it was increased significantly (24.8+/-3.1 vs. 29.3+/-2.9; P=0.011).'] Does regular physical activity increase glutathione peroxidase activity in adolescents with Down syndrome?