The clown knifefish is a facultative air breather, which is widely farmed in freshwater ponds in Vietnam. Here we report a very high nitrite tolerance (96 h LC50of 7.82 mM) in this species and examine the effectsof 1 mM (LC5) and 2.5 mM (LC10) ambient nitrite on haemoglobin (Hb) derivatives, electrolyte levels,acid-base status, and total body water content during 7 days of exposure. Furthermore, we tested thehypothesis that erythrocyte methaemoglobin (metHb) reductase activity is upregulated by nitrite expo-sure. Plasma nitrite levels increased for 2–3 days but stayed below environmental levels and fell towardscontrol values during the last half of the exposure period. Plasma nitrate, in contrast, rose continuously,reflecting detoxification of nitrite to nitrate. MetHb generated from the reaction between nitrite and ery-throcyte Hb reached 38% at day 2, but then decreased to 17% by the end of experiment. The first orderrate constant for metHb reduction by erythrocyte metHb reductase increased from 0.01 in controls to0.046 min−1after 6 days of nitrite exposure, showing up-regulation of this enzyme. While such upreg-ulation has been suggested in nitrite-exposed fish species, this study provides the first experimentalevidence.