The nitrite reductase activity of deoxyhemoglobin has received much recent interest because the nitric oxide produced in this reaction may participate in blood flow regulation during hypoxia. The present study used spectral deconvolution to characterize the reaction of nitrite with carp and rabbit hemoglobin at different constant oxygen tensions that generate the full range of physiological relevant oxygen saturations. Carp is a hypoxia-tolerant species with very high hemoglobin oxygen affinity, and the high R-state character and low redox potential of the hemoglobin is hypothesized to promote NO generation from nitrite. The reaction of nitrite with deoxyhemoglobin leads to a 1 : 1 formation of nitrosylhemoglobin and methemoglobin in both species. At intermediate oxygen saturations, the reaction with deoxyhemoglobin is clearly favored over that with oxyhemoglobin, and the oxyhemoglobin reaction and its autocatalysis are inhibited by nitrosylhemoglobin from the deoxyhemoglobin reaction. The production of NO and nitrosylhemoglobin is faster and higher in carp hemoglobin with high O2 affinity than in rabbit hemoglobin with lower O2 affinity, and it correlates inversely with oxygen saturation. In carp, NO formation remains substantial even at high oxygen saturations. When oxygen affinity is decreased by T-state stabilization of carp hemoglobin with ATP, the reaction rates decrease and NO production is lowered, but the deoxyhemoglobin reaction continues to dominate. The data show that the reaction of nitrite with hemoglobin is dynamically influenced by oxygen affinity and the allosteric equilibrium between the T and R states, and that a high O2 affinity increases the nitrite reductase capability of hemoglobin.