1. IntroductionAmmonia is the major metabolic end product during the catabolism of proteins, amino acids, and other nitrogen-containing biomolecules in various animal tissues. Ammonia is very toxic to fish. Its toxicity leads to reduced growth rate (Atwood et al., 2000; El-Shafai et al., 2004; Hegazi and Hasanein, 2010), disruption of ion-osmo homeostasis (Knoph and Thorud, 1996; Person-Le Ruyet et al., 2003, 1998), branchial hyperplasia (Benli et al., 2008), and if present in very high concentrations it causes hyperexcitability, coma, convulsions and finally death (Ip et al., 2001b). Due to ammonia toxicity, fish modify their metabolism by decreasing ammonia production, increasing ammonia excretion, or converting ammonia into glutamine and/or urea (Ip et al., 2001b). Most freshwater teleosts are ammoniotelic, excreting ammonia as a primary excretory product into the external environment mainly by diffusion through the gills (Saha and Ratha, 2007). However, several fish species have adapted to unique environmental circumstances by expressing high levels of OUC enzymes and thus converting more than 50% of waste nitrogen as urea-N, they are considered ureotelic (Anderson, 2001; Saha and Ratha, 2007) . However, some recent studies have proposed an alternative to ureothelialism (i.e. increased activity of the OUC pathway) as a mechanism to respond to such environmental circumstances. For example, in the marbled goby (Oxyeleotris marmoratus), a freshwater facultative breather, which can tolerate continuous exposure to air for up to a week, glutamine synthetase (GS) appears to function as an ammonia trap (Jow et al., 1999). A similar observation was made in swamp eel (Monopterus albus) concentrations (Tay et al.,...... middle of paper ......). Furthermore, evaporation of water 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 concentrations of EA (Rao et al., 1994). In the present study, multiple GS mRNA transcripts and their differential expression pattern are present in tissue air. Breathing walking catfish (C. batrachus) were studied during exposure to high ambient ammonia (HEA) ( 50 mM NH4Cl). Furthermore, attempts have been made for the characterization of several GS proteins, and for this integrated approaches of computational analysis and expression profiling have been used to predict properties and characteristics that might be important for their function and to elucidate its possible association with hyperammonia stress..