We have examined the electrochemical reduction of NO by myoglobin (Mb) contained within a dimethyldidodecylammonium bromide (ddab) Nm on pyrolytic graphite electrodes. Immersion of the (Fe-III)-Mb-doped electrode Into aqueous solutions of NO results in a bulk chemical reductive nitrosylation forming NO-Mb-Fe-II, as indicated by the dissappearance of the Fe-III/II-Mb couple in voltammograms. At more negative potentials, a catalytic reduction wave appears at ca. -0.7 V/SCE, which remains catalytic in solutions from pH 5.5 to pH IO and is NO-concentration dependent. Bull; electrolysis at -0.8 V/SCE of (NO)-N-15 solutions by Mb/ddab yields (N2O)-N-15 as gaseous product. Cyclic voltammograms of films made of preformed nitrosyl myoglobin, MbFe(II)-NO, demonstrate a single, reversible reduction to the nitroxyl state, MbFe(II)-NO-, at E degrees = -0.87 V/SCE. The reversibility of the nitrosyl reduction is pH dependent; digital simulation yields a rate of 22.5 s(-1) for the irreversible loss of a nitroxyl group at pH 7 and 0.7 s(-1) at pH 10. The catalytic formation of N2O during reduction of MbFe(II)-NO in the presence of exogenous NO implies that an N-N coupling reaction occurs at the active site between the Fe-bound nitroxyl and a free NO. A mechanism is proposed for the catalysis involving decomposition elf an (N2O2-)-Fe-II-Mb intermediate.