Nitric oxide (NO) has been shown to be the endothelium-derived relaxing factor (EDRF), and its impairment contributes to a variety of cardiovascular disorders. Recently, it has been recognized that nitrite can be an important source of NO; however, questions remain regarding the activity and mechanisms of nitrite bioactivation in vessels and its physiological importance. Therefore, we investigated the effects of nitrite on in vivo hemodynamics in rats and in vitro vasorelaxation in isolated rat aorta under aerobic conditions. Studies were performed to determine the mechanisms by which nitrite is converted to NO. In anesthetized rats, nitrite dose-dependently decreased both systolic and diastolic blood pressure with a threshold dose of 10 µM. Similarly, nitrite (10 µM - 2 mM) caused vasorelaxation of aortic rings, and NO was shown to be the intermediate factor responsible for this activity. Using electrochemical, as well as electron paramagnetic resonance (EPR) spectroscopy techniques, NO generation was measured from isolated aortic vessels following nitrite treatment. Reduction of nitrite to NO was blocked by heating the vessel, suggesting that an enzymatic process is involved. Organ chamber experiments demonstrated that aortic relaxation induced by nitrite could be blocked by both hemoglobin and soluble guanylyl cyclase (sGC) inhibitor ODQ. In addition, both electrochemical and EPR spin trapping measurements showed that ODQ inhibits nitrite-mediated NO production. These findings, thus, suggest that nitrite can be a precursor of EDRF and that sGC or other heme proteins inhibited by ODQ catalyze the reduction of nitrite to NO.