Oxidation of oxyhemoglobin by nitrite is characterized by a lag period followed by an autocatalytic phase. The oxidation can be inhibited by the addition of morpholine, piperidine, triethanolamine or triethylamine (6 mM each). These amines are known to react with nitrogen dioxide to yield nitrosamine. Unexpectedly, aniline or aminopyrine (120 microM each) markedly inhibited the oxidation. These compounds, but not the other amines given above, inhibited the peroxide compound formation from methemoglobin and hydrogen peroxide. The results establish that, during the oxidation, the peroxide compound is generated and converts nitrite into nitrogen dioxide by its peroxidatic activity, resulting in an autocatalytic phase.