Recent evidence suggests that the reaction between nitrite and deoxygenated hemoglobin provides a mechanism by which nitric oxide is synthesized in vivo. This reaction has been previously defined to follow second order kinetics, although variable product stoichiometry has been reported. In this study we have re-examined this reaction and found that under fully deoxygenated conditions the product stoichiometry is 1:1 (methemoglobin:nitrosylhemoglobin), and unexpectedly, the kinetics deviate substantially from a simple second order reaction and exhibit a sigmoidal profile. The kinetics of this reaction are consistent with an increase in reaction rate elicited by heme oxidation and iron-nitrosylation. In addition, conditions that favor the "R" conformation show an increased rate over conditions that favor the "T" conformation. The reactivity of nitrite with heme is clearly more complex than has been previously realized and is dependent upon the conformational state of the hemoglobin tetramer, suggesting that the nitrite reductase activity of hemoglobin is under allosteric control.