The role of hemoglobin in transporting oxygen is dependent on the reversible binding of oxygen to Fe(II) hemoglobin with molecular oxygen released at reduced oxygen pressures. The partially oxygenated hemoglobin formed with the release of oxygen from hemoglobin is susceptible to redox reactions where the functional Fe(II) heme is oxidized to Fe(III) and the substrate is reduced. In this article, we review two important redox reactions of hemoglobin and discuss the ramifications of these reactions. The reduction of oxygen to superoxide starts a cascade of oxidative reactions, which are a source for red cell-induced oxidative stress. The reduction of nitrite to nitric oxide produces a labile form of nitric oxide that can be a source for oxidative stress, but can also have important physiological functions.