Nitric oxide (NO^•) is thought to protect against the damaging effects of myocardial ischemia–reperfusion injury, whereas xanthine oxidoreductase (XOR) normally causes damage through the generation of reactive oxygen species. In the heart, inorganic nitrite \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}({\mathrm{NO}}_{2}^{-})\end{equation*}\end{document} has the potential to act as an endogenous store of NO^•, liberated specifically during ischemia. Using a detection method that we developed, we report that under ischemic conditions both rat and human homogenized myocardium and the isolated perfused rat heart (Langendorff preparation) generate NO^• from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{NO}}_{2}^{-}\end{equation*}\end{document} in a reaction that depends on XOR activity. Functional studies of rat hearts in the Langendorff apparatus showed that nitrite (10 and 100 μM) reduced infarct size from 47.3 ± 2.8% (mean percent of control ± SEM) to 17.9 ± 4.2% and 17.4 ± 1.0%, respectively (P < 0.001), and was associated with comparable improvements in recovery of left ventricular function. This protective effect was completely blocked by the NO^• scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide (carboxy-PTIO). In summary, the generation of NO^• from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{NO}}_{2}^{-}\end{equation*}\end{document}, by XOR, protects the myocardium from ischemia–reperfusion injury. Hence, if XOR is presented with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{NO}}_{2}^{-}\end{equation*}\end{document} as an alternative substrate, the resultant effects of its activity may be protective, by means of its production of NO^•, rather than damaging.