Exposure of Siberian sturgeon (Acipenser baeri) yearlings (172.0±18 g; mean±S.D.) to several NO2-¯N concentrations (0, 25, 130, 180 and 275 mg/l) was studied for 72 h in static tests. At 72 h, the median-lethal concentration of NO2-¯N was 130 mg/l in water with high chloride content (130.5 mg/l). Nitrite exposure produced high levels of methaemoglobin (MetHb) but did not seem to cause mortality, as surviving fish showed higher levels (82.7±5.6%) than torpid specimens (60.8±4.5%). Levels of MetHb were unrelated to environmental and plasmatic nitrite concentrations, as both torpid and surviving fish exposed to the highest nitrite levels (275 mg/l of NO2-¯N) presented similar concentrations of MetHb to those exposed to 25 mg/l of NO2-¯N, thus indicating the ability of Siberian sturgeon yearlings to regulate plasmatic nitrite levels and maintain them lower than the environmental concentration of the toxicant. Nitrite exposure caused changes in the plasmatic electrolyte balance, which is characterised by extracellular hyperkalemia, high plasmatic chloride levels and low plasmatic sodium concentration. Differences between the nitrite concentration in the liver of torpid (46.3±9.0 mg/l) and surviving specimens (19.1±13.1 mg/l) exposed to several concentrations of NO2-¯N suggest a significant contribution of the liver in nitrite detoxification pathways, and would thus explain a possible nitrite tolerance of Siberian sturgeon yearlings.