The 24-, 48- and 96-h LC50 of nitrite and the effects on reactive oxygen intermediates (ROIs) in haemocytes and muscle, superoxide dismutase (SOD) activity, catalase (CAT) activity, glutathione peroxidase (GPX) activity, in muscle of Macrobrachium nipponense exposed to ambient nitrite were investigated. The 24-, 48- and 96-h LC50 values of nitrite at pH 8.0 were 46, 26 and 13.3 mg l-1. A significantly increased ROIs value was found in prawn (P<0.05) exposed to nitrite shock and increased with increased ambient nitrite concentration. Exposure to a sublethal concentration of nitrite for 24 h, increased ROIs by 97% compared with that of prawn exposed to control nitrite concentration, whereas SOD, CAT and GPX activity decreased 67%, 80.6% and 82.7%, respectively. The results showed that an imbalance between prooxidant forces and antioxidant defenses is one of the toxicity mechanisms of nitrite on prawn.