Ferrihemoproteins in buffer solutions bind nitric oxide to yield their nitric oxide adducts. Reversible binding of NO was found for ferricytochrome c (Cyt(III)) and metmyoglobin (Mb(III)) at pH values lower than ca. 7.0. The equilibrium constants were obtained as (1.6 +/- 0.1) x 10(4) M(-1) for Cyt(III) and (1.3 +/- 0.1) x 10(4) M(-1) for Mb(III). At higher pH, the reversible formation of the NO adducts is no longer observed; the NO adduct of Cyt(III) (Cyt(III)-NO) undergoes reduction to ferrocytochrome c, Cyt(II), and that of Mb(III) (Mb(III)-NO) to the nitrosyl adduct of Mb(II) (Mb(II)-NO). Methemoglobin (Hb(III)) reacts readily with NO even at pH < 6 to give the nitrosyl adduct of hemoglobin (Hb(II)-NO). The rates for the formation of Cyt(II), Mb(II)-NO, and Hb(II)-NO were measured as functions of NO and OH- concentrations. Kinetic analysis indicates that Cyt(III)-NO and Mb(III)-NO undergo nucleophilic attack by OH- at higher pH to yield Cyt(II) and Mb(II), respectively. Mb(II) thus produced further reacts with NO to give Mb(II)-NO. For Hb(III), the nitrosyl adduct (Hb(III)-NO) was found to react with both OH- and H2O to give Hb(II)-NO in the presence of excess NO. The rate constants for the reaction between the nitrosyl ferrihemoproteins and OH- were determined as (1.5 +/- 0.1) x 10(3) M(-1) s(-1) for Cyt(III)-NO, (3.2 +/- 0.2) x 10(2) M(-1) s(-1) for Mb(III)-NO, and (3.2 +/- 0.2) x 10(3) M(-1) s(-1) for Hb(III)-NO. The reductive nitrosylation of Hb(III) observed at pH < 6.0 is explained by reaction of H2O with Hb(III)-NO : the rate constant is (1.1 +/- 0.1) x 10(-3) s(-1).