Abstract The effect of hypoxia and reoxygenation on Nile tilapia (Oreochromis niloticus) was studied. The fish were exposed to hypoxic oxygen concentrations (2 and 0.5 mg O2 L-1) for 1 day (short term) and 30 days (long term) followed by reoxygenation for 1 day and 7 days, respectively. The activities of superoxide dismutase (SOD), catalase (CAT) and glutathione S-transferase (GST) significantly decreased in the gills of all the hypoxic groups after both short and long term of hypoxia exposures, while increased significantly below control levels after reoxygenation except that the activities of SOD and GST in the reoxygenated groups pre-exposed to short-term (2 mg O2 L-1), SOD activity in the reoxygenated group pre-exposed to long-term (0.5 mg O2 L-1), and the activities of CAT and GST in all the reoxygenated groups pre-exposed to long-term hypoxia, had no significant change compared to their respective hypoxic groups. The activities of glutathione peroxidase (GPx), glutathione reductase (GR) and xanthine oxidase (XO) significantly increased during hypoxia then decreased significantly after reoxygenation except for GPx activity in the reoxygenated group pre-exposed to short-term (2 mg O2 L-1). Malondialdehyde (MDA) concentrations significantly increased in all the hypoxic and reoxygenated groups compared to their respective controls. The significance of these alterations in enzyme activities and oxidative stress indices is discussed. In conclusion, hypoxia and subsequent reoxygenation induce oxidative stress in Oreochromis niloticus gills causing depletion of some antioxidant enzymes. Long-lasting oxidative stress can delay full recovery. Key words: Antioxidant enzymes, gills, hypoxia, reoxygenation, Oreochromis niloticus.					ARTICLE TOOLS Abstract PDF Fulltext How to cite this article Citation Tools Related Records   Articles by Zeinab Ibrahim Attia Articles by Hala Mostafa Abdelazem Articles by Iman Elsayed Nada on Google on Google Scholar