S-nitrosothiols (SNO) perform many important functions in biological systems, but the mechanism by which they are generated in vivo remains a contentious issue. Nitric oxide (NO) reacts with thiols to form SNO only in the presence of a molecule that will accept an electron from either NO or the thiol. In this study, we present evidence that ferriheme accepts an electron from NO or glutathione (GSH) to generate S-nitrosoglutathione (GSNO) in vitro under anaerobic or hypoxic (2% O2) conditions. Ferriheme formed charge transfer-stable complexes with NO to form ferriheme-NO (heme-Fe(II)-NO+) and with GSH to form ferriheme-GS (heme-Fe(II)-GS•) under anaerobic conditions. The reaction between GSH and the heme-Fe(II)-NO+ complex or between NO and the heme-Fe(II)-GS• complex resulted in simultaneous reductive ferriheme nitrosylation (heme-Fe(II)NO) and the generation of GSNO. Thus, ferriheme is readily reduced to ferroheme in the presence of NO and GSH together, but not with either individually. The reaction between NO and the heme-Fe(II)-GS• complex to generate GSNO occurred more rapidly than NO was consumed by endothelial cells, but not red blood cells. In addition, pretreatment of endothelial cells with ferriheme or the ferriheme-GS complex generated SNO upon addition of NO under hypoxic conditions. The results of this study raise the possibility that in vivo, ferriheme can complex with GSH to form ferriheme-GS complex (heme-Fe(II)-GS•), which rapidly reacts with NO to generate GSNO under intracellular oxygen levels. The GSNO formation by this mechanism is more efficient than any other in vitro mechanism(s) reported so far.