SUMMARY The objective of this study was to elucidate if chronic and acute ammonia intoxication in mudskippers, Periophthalmodon schlosseri and Boleophthalmus boddaerti, were associated with high levels of ammonia and/or glutamine in their brains, and if acute ammonia intoxication could be prevented by the administration of methionine sulfoximine [MSO; an inhibitor of glutamine synthetase (GS)] or MK801 [an antagonist of n-methyl d-aspartate type glutamate (NMDA) receptors]. For P. schlosseri and B. boddaerti exposed to sublethal concentrations(100 and 8 mmol l-1 NH4Cl, respectively, at pH 7.0) of environmental ammonia for 4 days, brain ammonia contents increased drastically during the first 24 h, and they reached 18 and 14.5 μmol g-1,respectively, at hour 96. Simultaneously, there were increases in brain glutamine contents, but brain glutamate contents were unchanged. Because glutamine accumulated to exceptionally high levels in brains of P. schlosseri (29.8 μmol g-1) and B. boddaerti (12.1μmol g-1) without causing death, it can be concluded that these two mudskippers could ameliorate those problems associated with glutamine synthesis and accumulation as observed in patients suffering from hyperammonemia. P. schlosseri and B. boddaerti could tolerate high doses of ammonium acetate (CH3COONH4)injected into their peritoneal cavities, with 24 h LC50 of 15.6 and 12.3 μmol g-1 fish, respectively. After the injection with a sublethal dose of CH3COONH4 (8 μmol g-1fish), there were significant increases in ammonia (5.11 and 8.36 μmol g-1, respectively) and glutamine (4.22 and 3.54 μmol g-1, respectively) levels in their brains at hour 0.5, but these levels returned to normal at hour 24. By contrast, for P. schlosseriand B. boddaerti that succumbed within 15-50 min to a dose of CH3COONH4 (15 and 12 μmol g-1 fish,respectively) close to the LC50 values, the ammonia contents in the brains reached much higher levels (12.8 and 14.9 μmol g-1,respectively), while the glutamine level remained relatively low (3.93 and 2.67 μmol g-1, respectively). Thus, glutamine synthesis and accumulation in the brain was not the major cause of death in these two mudskippers confronted with acute ammonia toxicity. Indeed, MSO, at a dosage(100 μg g-1 fish) protective for rats, did not protect B. boddaerti against acute ammonia toxicity, although it was an inhibitor of GS activities from the brains of both mudskippers. In the case of P. schlosseri, MSO only prolonged the time to death but did not reduce the mortality rate (100%). In addition, MK801 (2 μg g-1 fish) had no protective effect on P. schlosseri and B. boddaerti injected with a lethal dose of CH3COONH4, indicating that activation of NMDA receptors was not the major cause of death during acute ammonia intoxication. Thus, it can be concluded that there are major differences in mechanisms of chronic and acute ammonia toxicity between brains of these two mudskippers and mammalian brains.