Abstract: This study was performed to investigate whether differences exist in activities of Glutathione Peroxidase (GPx), Glutathione Reductase (GSR) and Glutathione S-transferase (GST) in hearts between Tibet chicken and a lowland chicken breed (Silky chicken). At the end of 5 days of age, 24 chicklings of each breed were divided into 3 groups treated with three different oxygen concentrations, respectively for 20 h. Activities of the three enzymes in chicken hearts were determined spectrophotometrically.