1. IntroductionAmmonia is the major metabolic end product during the catabolism of proteins, amino acids, and other nitrogen-containing biomolecules in different animal tissues. Ammonia is very toxic to fish. Its toxicity leads to a reduction in growth rate (Atwood et al., 2000; El-Shafai et al., 2004; Hegazi and Hasanein, 2010), a disruption of ionic osmosis homeostasis (Knoph and Thorud, 1996 ; Person-Le Ruyet et al., 2003, 1998), gill hyperplasia (Benli et al., 2008), and if present at very high concentrations, it causes hyperexcitability, coma, convulsions and ultimately death (Ip et al., 2001b).To survive the effect of Due to the toxicity of ammonia, fish modify their metabolism either by decreasing ammonia production, increasing its excretion, or by converting ammonia into glutamine and/or urea (Ip et al., 2001b). Most freshwater teleosts are ammoniotelic, as they excrete ammonia as the main excretory product to the external environment, mainly by diffusion through the gills (Saha and Ratha, 2007). But several fish species have adapted to unique environmental circumstances by expressing high levels of OUC enzymes and thereby converting more than 50% of waste nitrogen to urea-N. They are considered ureotelic (Anderson, 2001; Saha and Ratha, 2007). ). However, many recent studies have proposed an alternative to ureotelism (i.e., increased OUC pathway activity) as a mechanism to respond to such environmental circumstances. For example, in the marbled goby (Oxyeleotris marmoratus), a facultative freshwater air breather, which can tolerate continuous exposure to air for up to a week, glutamine synthetase (GS) appears to function as a ammonia trap (Jow et al., 1999). A similar observation was made in concentrations of marsh eels (Monopterus albus) (Tay et al.,...... middle of paper......). Additionally, water evaporation at high temperatures in the tropics can concentrate external ammonia (EA) (Rao et al., 1994). The situation is further aggravated for fish living in rice fields, where agricultural fertilization can lead to high EA concentrations (Rao et al., 1994). In the present study, the presence of several GS mRNA transcripts and their differential expression pattern in atmospheric tissues. -breathing catfish (C. batrachus) during exposure to high environmental ammonia (HEA) (NH4Cl 50 mM) were studied. Furthermore, attempts have been made to characterize different GS proteins, and for this, integrated computational analysis and expression profiling approaches have been used to predict properties and features that may be important for their function and for elucidate its possible association with hyperammonia stress..