Chinese loaches (body weight:18-25 g) were exposed to 30 mmol/L NH₄Cl solution and air (exposure time:0, 6, 12, 24, 48, 72 h) to assess changes in tissue glutamine content, glutamine synthetase (GS) activity, and glutamate dehydrogenase (GDH) activity during ammonia loading and aerial exposure. Glutamine accumulation in liver and muscle, as well as GS activity in the brain, liver, and intestine, were observed with increasing duration of ammonia and aerial exposure. In all analyzed tissue except the liver, GDH activity was significantly affected by ammonia and aerial exposure. Our results suggested that Chinese loaches respond to internal ammonia increase through glutamine accumulation. Glutamine synthetase then stimulates the glutamine formation pathway and converts ammonia into non-toxic glutamine. The marked increase of GDH activity in the intestines demonstrated that intestinal GDH is more important than intestinal GS in the overall defense against ammonia toxicity. Across the exposure period, variation in liver GDH activity was not affected by ammonia and air, nor associated with a concomitant increase in GS activity and glutamine content that probably occurred due to efficient glutamine replenishment via transaminase action.