This study provides threshold concentrations at which metals cause effects on the antioxidant system in Eagle owls. Blood Cd concentrations greater than 0.3 mu g/dl produced an inhibition in GPx (32%) and CAT (26%) activity in RBC. However, Cd concentrations higher than 0.02 mu g/dl were enough to produce an inhibition of these enzymes. Regarding Pb levels, blood concentrations above 2 mu g/dl produced an inhibition of 8% and 10.5% in GPx and CAT activities, respectively, in RBC. A depletion of 16% and 4% in tGSH levels was associated with Pb concentrations higher than 15 and 3 mu g/dl, respectively, in individuals from the ancient mine site. In addition, Pb concentrations above 2 and 10 mu g/dl produced a TBARS induction of 10% and 28%, respectively, in individuals from both the industrial and the mining area. Finally, Hg concentrations greater than 3 and 10 mu g/dl resulted in a TBARS induction of 102% and 190%, respectively, in Eurasian eagle owls from the industrial area. Our findings show that Pb may produce effects on oxidative stress biomarkers in Strigiformes at lower concentrations than those typically accepted for considering physiological effects in Falconiformes (20 mu g/dl in blood). In addition, we provide new data on Hg and Cd concentrations related to effects in the antioxidant system. (C) 2014 Elsevier Inc. All rights reserved.