After nonionic ammonia stress(0.1 mg/L and 0.5 mg/L),changes of activities of hexokinase(HK),pyruvate kinase(PK),lactic dehydrogenase(LDH),and succinate dehydrogenase(SDH)were compared between L.vannamei long-term cultured in freshwater and in seawater.Results showed that:(1)nonionic ammonia in lower concentration induced the HK activity in gills of freshwater shrimps to a significantly higher level,while nonionic ammonia in higher concentration induced a significantly higher level of HK activity in gills of freshwater and seawater shrimps.After stress,no significant change was observed in HK activity in muscles of freshwater and seawater shrimps.(2)After nonionic ammonia stress,an increase was observed in PK activity in gills of all shrimps,and PK activity returned to the normal level at the end of the experiments.However,PK activity in muscles showed a different change:no significant change was found in seawater shrimps,which was different from that in freshwater shrimps.(3)No significant effects of nonionic ammonia in lower concentration on LDH activity in gills and muscles of freshwater and seawater shrimps were observed.In contrast,nonionic ammonia in higher concentration imposed a significant effect on LDH activity in gills and muscles of freshwater shrimps.(4)SDH activity was significantly lower in both seawater and freshwater after nonionic ammonia stress.We conclude that nonionic ammonia stress has a significant effect on respiratory metabolic enzyme activities of L.vannamei,and aerobic metabolism decreased drastically after nonionic ammonia stress,while anaerobic metabolism increased slightly in the initial period prior to decreasing.It was suggested that nonionic ammonia probably led to a change in main energetic materials in shrimps.