The present study explored the role of myoglobin (Mb) in cardiac NO homeostasis and its functional relevance by employing isolated hearts of wild-type (WT) and myoglobin knockout mice. ¹H NMR spectroscopy was used to measure directly the conversion of oxygenated Mb (MbO₂) to metmyoglobin (metMb) by reaction with NO. NO was applied intracoronarily (5 nM to 25 μM), or its endogenous production was stimulated with bradykinin (Bk; 10 nM to 2 μM). We found that infusion of authentic NO solutions dose-dependently (≥ 2.5 μM NO) increased metMb formation in WT hearts that was rapidly reversible on cessation of NO infusion. Likewise, Bk-induced release of NO was associated with significant metMb formation in the WT (≥1 μM Bk). Hearts lacking Mb reacted more sensitively to infused NO in that vasodilatation and the cardiodepressant actions of NO were more pronounced. Similar results were obtained with Bk. The lower sensitivity of WT hearts to changes in NO concentration fits well with the hypothesis that in the presence of Mb, a continuous degradation of NO takes place by reaction of MbO₂ + NO to metMb + NO₃⁻, thereby effectively reducing cytosolic NO concentration. This breakdown protects myocytic cytochromes against transient rises in cytosolic NO. Regeneration of metMb by metMb reductase to Mb and subsequent association with O₂ leads to reformation of MbO₂ available for another NO degradation cycle. Our data indicate that this cycle is crucial in the breakdown of NO and substantially determines the dose–response curve of the NO effects on coronary blood flow and cardiac contractility.