B-type natriuretic peptide (BNP) has been reported to be released from the myocardium during ischemia. We hypothesized that BNP mediates cardioprotection during ischemia-reperfusion and examined whether exogenous BNP limits myocardial infarction and the potential role of ATP-sensitive potassium (K_ATP) channel opening. Langendorff-perfused rat hearts underwent 35 min of left coronary artery occlusion and 120 min of reperfusion. The control infarct-to-risk ratio was 44.8 ± 4.4% (means ± SE). BNP perfused 10 min before ischemia limited infarct size in a concentration-dependent manner, with maximal protection observed at 10⁻⁸ M (infarct-to-risk ratio: 20.1 ± 5.2%,P < 0.01 vs. control), associated with a 2.5-fold elevation of myocardial cGMP above the control value. To examine the role of K_ATP channel opening, glibenclamide (10⁻⁶ M), 5-hydroxydecanoate (5-HD; 10⁻⁴ M), or HMR-1098 (10⁻⁵ M) was coperfused with BNP (10⁻⁸ M). Protection afforded by BNP was abolished by glibenclamide or 5-HD but not by HMR-1098, suggesting the involvement of putative mitochondrial but not sarcolemmal K_ATP channel opening. We conclude that natriuretic peptide/cGMP/K_ATPchannel signaling may constitute an important injury-limiting mechanism in myocardium.