Although numerous reports have documented that the S-nitrosylation of cysteine residues by NO alters the activities of a wide variety of proteins, the direct visualization and the structural consequences of this reversible modification have not yet been reported for any protein. Here we describe the crystal structure of S-nitroso-nitrosylhemoglobin determined at a resolution of 1.8 Å. The specific reaction of NO with Cys93β is confirmed in this structure, and a large S-nitrosylation-induced change in the tertiary structure of the COOH-terminal dipeptides of the β subunits provides additional insight into the stereochemical mechanism by which blood flow is regulated by the interaction of NO with hemoglobin.