At birth, premature infants are suddenly exposed to hyperoxic conditions compared to intra-uterine conditions. The antioxidant enzym (AOE) system (SOD, catalase and glutathione peroxidase), is not expected to be fully developed at early gestational ages. Therefore there is a higher risk of oxydative tissue damage. This study was designed to test whether hyperoxia can induce AOE activity in selected organs of the developing chick embryo. Methods: Eggs from the White Leghorn were incubated and divided in four groups: one control group (n=100) and three test groups exposed during 48 hours at 60% O2 at day 10 (n=80), 14 (n=60) and 18 (n=30). Total-Superoxide-Dismutase (SOD) activity was measured in homogenates of the brain, heart, liver, intestines and lungs from the 10th until the 21th day of the incubation in the control group (10 each day), and in the test groups (10 each day) 24 hours and each 48 hours post hyperoxia exposure. Catalase and glutathione peroxidase activity are not reported here. Results: The SOD activity increased in all selected organs, except the brain, during the development of the chick embryo. Exposure to hyperoxia at different incubation times showed a two to ten fold increase in SOD activity after 48 hours. This significant increased enzyme activity disappeared after 72 hours.Conclusion: Exposure to hyperoxia during 48 hours at different incubation time induces a similar increase in SOD activity. This might serve as a protective mechanism to a relatively hyperoxic environment.