It is known that peroxidase oxidation of nitrite to toxic products is one of the basic mechanisms of its toxicity. Therefore, the ratio between activities of hydrogen peroxide metabolized enzymes and the impact of nitrite on these enzymes is an important factor determining its toxicity. By use of calorimetric method developed by us that makes possible to control H2O2 destroying kinetics and distinguish different pathways of its destruction, we established that nitrite even in micromolar concentrations significantly decreases the ability of catalase to destroy hydrogen peroxide in the presence of chloride, bromide and thiocyanate, but not without them. Also it was established that this decrease is the result of reversible inhibition of the enzyme without any change in the essence of H2O2 destroying pathway. Mechanisms of the inhibition are discussed. Heme-containing peroxidases, such as horseradish peroxidase, lactoperoxidase and methemoglobin, manifested multiple higher stability towards nitrite inhibition than catalase does independently without halides and thiocyanate. Therefore, the presence of Cl-, Br- and SCN- in nitrite, catalase- and peroxidasecontaining systems should lead to increase the peroxidase pathway of H2O2 destroying and, consequently, the peroxidase oxidation of nitrite to a toxic product (probably NO2 • ). This finding was demonstrated in the example of nitrite-induced hemoglobin oxidation. Low anion concentration in intracellular media and also the presence of effective nitrite competitors for peroxidases appear to be the most important mechanism of cell protection from nitrite toxicity.