Similar responses were observed in o. mykiss for ALT (sanchez-Muros et al., 1998) and in R.qurlen for AST and ALT ( Melo et al., 2006). The rise in the hepatic activity of protein-metabolizing enzymes when fish were fed the 45%-CP diet may denote use of excess dietary amino acids for growth as well as substrate for gluconeogenesis, particularly for AST and ALT activities .Physiologica alteration were significantly affected by diet composition and weights of Nile tilapia and cold by used as a metabolic tool for assessing the proper concentration of dietary protein in the feeding of Nile tilapia. Moreover, the proper ratio of protein/ carbohydrate in the diet is fundamental to establish the optimal nitrogen content in the diet, to increase fish gain efficiency while preventing nitrogen waste and environmental damage. the ratio of protein/ energy in the Nile tilapia diet seems to be specific and furher studies concerning types and sources of carbohydrates and lipids to replace protein as energy sources are necessary.