Inhibition of K_ATP channel activity has previously been demonstrated to result in coronary vasoconstriction with decreased myocardial blood flow and loss of phosphocreatine. This study was performed to determine whether the high energy phosphate abnormality during K_ATP channel blockade can be ascribed to oxygen insufficiency. Myocardial blood flow and oxygen extraction were measured in open-chest dogs during K_ATP channel blockade with intracoronary glibenclamide, while high energy phosphates were examined with ³¹P-magnetic resonance spectroscopy (MRS) and myocardial deoxymyoglobin (Mb-) was determined with ¹H-MRS. Glibenclamide resulted in a 20±8% decrease of myocardial blood flow that was associated with loss of phosphocreatine (PCr) and accumulation of inorganic phosphate. Mb- was undetectable during basal conditions but increased to 58±5% of total myoglobin during glibenclamide administration. This degree of myoglobin desaturation during glibenclamide was far greater than we previously observed during a similar reduction of blood flow produced by a coronary stenosis (22% of myoglobin deoxygenated during stenosis). The findings suggest that reduction of coronary blood flow with an arterial stenosis was associated with a decrease of myocardial energy demands, and that this response to hypoperfusion was inhibited by K_ATP channel blockade.