The major pathway for nitric oxide scavenging in red cells involves the direct reaction of the gas with HbO₂ to form nitrate and the ferric form of the protein, metHb. Because both atoms of O₂ are incorporated into nitrate, this process is called NO dioxygenation (NOD). The NOD reaction involves an initial, very rapid bimolecular addition of NO to bound O₂ to form a transient Fe(III)–peroxynitrite complex, which can be observed spectrally at alkaline pH. This intermediate rapidly isomerizes at pH 7 (t_1/2 ≤ 1 ms) to metHb and NO₃⁻, which is nontoxic and readily transported out of red cells and excreted. The rate of NO consumption by intracellular HbO₂ during normal blood flow is limited by diffusion up to and into the red cells and is too slow to interfere significantly with vasoregulation. In contrast, extracellular HbO₂ is highly vasoconstrictive, and the resultant hypertension is a significant side effect of most hemoglobin-based blood substitutes. The major cause of this blood pressure effect seems to be the high rate of NO dioxygenation by cell-free HbO₂, which can extravasate into the vessel walls and interfere directly with NO signaling between endothelial and smooth muscle cells. This interpretation is supported by a strong linear correlation between the magnitude of the blood pressure effect caused by infusion of cross-linked recombinant hemoglobin tetramers in vivo and the rate of NO dioxygenation by these proteins measured in vitro.