Glutathione Peroxidase (GPx) family of enzymes plays important roles in the protection of organisms from oxidative damage. GPx converts reduced glutathione (GSH) to oxidized glutathione (GSSG) while reducing lipid hydroperoxides to their corresponding alcohols or free hydrogen peroxide to water. This study was conducted to investigate the possible effects of temperature in vivo on Glutathione peroxidase (GPX) oxidative activity in fish tissues.An adult fully grown cat fish, (the African flathead catfish, Clariasgariepinus) about 0.7m in length and 0.15 in width, weighing 15Kg was obtained from the school pond. The fish was humanely killed and its liver was cut off from the rest of the body. Glutathione peroxidase was extracted and purified from the liver of the fish using the purification method of Ezeet al(2010).The extracted and purified GPX was then divided into 23 test tubes and labeled. Each test tube was heated at different temperatures ranging from -30°C to 50°C, after which Glutathione and Hydrogen Peroxide substrates were introduced to each test tube.Results showed that measurements were higher in test tubes were heating temperature were either exactly at or close to the acclimation temperature of the African Flathead Catfish.