Blood glutathione peroxidase (GSH-Px; E.C. 1.11.1.9) activity, a measure of selenium status, as well as the plasma indicators of muscle damage creatine kinase (CK; E.C. 2.7.3.2) and aspartate aminotransferase (AST; E.C. 2.6.1.1) were evaluated in sheep in a region with low selenium levels during the spring and the summer, because this time may be critical for suffering selenium deficiencies. GSH-Px activity was highest during the spring, when mean values ranged from 74.16 and 90.51 Ul/g Hb, descending drastically during the summer, below physiological levels (P < 0.001) with mean values ranging from 43.33 to 50.01 Ul/g Hb. GSH-Px activity, within the deficiency range, registered during the summer did not originate muscle damage, since the muscle enzymes CK and AST were within their normal range.