The influence of small doses of exogenic nitrite on reversible and irreversible oxidative modifica tions of watersoluble proteins of rat cardiac and skeletal muscle was studied with the aid of redox 2Delec trophoresis and colorimetric determination of protein carbonyl group, correspondingly. To explain the absence of significant changes under hypoxia induced by nitrite the known hypothesis about nitrite inhibition of some sites of mitochondrial electron transporting chain decreasing free radical quantity was discussed.