SUMMARY Nitrite is a potential nitric oxide (NO) donor and may have important biological functions at low concentrations. The present study tests the hypothesis that nitrite accumulation across the gills in fish will cause a massive NO production from nitrite. Zebrafish were exposed to three different nitrite levels for variable time periods, and changes in blood nitrosylhemoglobin (HbNO), methemoglobin (metHb), oxygenated hemoglobin(oxyHb) and deoxygenated hemoglobin (deoxyHb) were evaluated by spectral deconvolution. Blood HbNO (a biomarker of internal NO production) was low in controls, increased to a stable level around 3.7% of total Hb in fish exposed to 0.6 mmol l–1 nitrite, and to 12.1% (at day 2) in fish exposed to 2 mmol l–1 nitrite. The very high HbNO levels testify to an extensive conversion of nitrite to NO. With deoxyHb-mediated reduction of nitrite being a major NO-producing mechanism, the data reveal the significance of this mechanism, when hemoglobin cycles between full and intermediate oxygen saturations in the arterial–venous circulation. Fish exposed to 0.6 mmol l–1 nitrite for up to 5 days could be divided into responding (with elevated metHb) and non-responding individuals. Exposure to 2 mmol l–1 nitrite caused a time-dependent increase in metHb to 59% of total Hb within 2 days. Taking HbNO into account,the functional (potential O2 carrying) Hb was reduced to 29% at this stage. Total blood [Hb] was also significantly decreased. In spite of the reduced blood O2 capacitance, and the possibility that excess NO may inhibit mitochondrial respiration, whole animal routine oxygen consumption was not depressed.