It has been suggested that nitrosyliron(II)hemoglobin may represent a form of stabilized NO. and may be responsible for NO, delivery in the peripheral circulation. In this work, we show that NO, can be released from nitrosyliron(II)hemoglobin through reaction with peroxynitrite. Outer-sphere oxidation of the iron center generates nitrosyliron(III)hemoglobin, from which NO. dissociates at a rate of ca. 1 s(-1). The second-order rate constant for the reaction of peroxynitrite with nitrosyliron(II)hemoglobin is (6.1 +/- 0.3) x 10(3) M-1 s(-1) (at pH 7.2 and 20 degreesC). In the presence of 1.2 MM CO2, the rather large value of the second-order rate constant, (5.3 +/- 0.2) x 10(4) M-1 s(-1) (at pH 7.2 and 20 degreesC), indicates that this reaction may take place in vivo. The reactive nitrogen species generated from this reaction, N2O3 and/or NO2, may lead to protein modifications, such as nitration of tyrosine and/or tryptophan residues and nitrosation of cysteine residues.