Nitrosylhemoglobin can be produced by introducing gaseous NO into an aqueous solution of hemoglobin. It has been demonstrated that nitrosylhemoglobin in aqueous solution can be converted to SNO-hemoglobin upon introduction of oxygen to the solution, as is postulated to occur in the lungs. Nitrosylhemoglobin can be used in methods to produce the physiological effects of NO, for example, to reduce vasoconstriction and to inhibit platelet aggregation.