In heart, glycolysis may be a preferential source of adenosine triphosphate (ATP) for membrane functions. In this study the patch-clamp technique was used to study potassium channels sensitive to intracellular ATP levels in permeabilized ventricular myocytes. Activation of these K⁺ channels has been implicated in marked cellular K⁺ loss leading to electrophysiological abnormalities and arrhythmias during myocardial ischemia. The results showed that glycolysis was more effective than oxidative phosphorylation in preventing ATP-sensitive K⁺ channels from opening. Experiments in excised inside-out patches suggested that key glycolytic enzymes located in the membrane or adjacent cytoskeleton near the channels may account for their preference for glycolytic ATP.