Glutamine synthetase (GS) plays important roles in plants like assimilation of ammonium and detoxification of the ammonium released from many metabolic processes such as amino acid degradation or photorespiration. Using ATP, ammonia is combined with glutamate to yield glutamine by the action of GS. Phosphinothricin (PPT) is widely used as a herbicide because it competes with glutamate to bind the active site of GS. PPT has been used to produce transgenic Brachypodium distachyon callus and plants as a selectable agent. PPT treatment raises ammonium content and induces toxicity in non-transformed cells. To find out efficient condition for selecting transformed callus, ammonium content were measured in this study. Non-transformed callus were derived from mature seeds of Brachypodium distachyon (Bd21). The callus were cultured on the callus inducing media (CIM) or regeneration media (RM) containing serial dilutions of the PPT (2, 5, 10 and 15 mg/l) with or without light. Ammonium content was measured 2 weeks after PPT application. Ammonium toxicity associated with PPT treatment was dose-dependent on RM whereas PPT treatment was not significantly influenced on CIM. There is no influence on dark or light condition. Additionally, callus were cultured on the media containing phytohormones combined with PPT (5 mg/l) and the most affecting element causing increased ammonium content has been identified. Acknowledgements: This work was supported by a grant (No. 20070301-034-016-007) from BioGreen 21 Program, RDA, Republic of Korea.