The present studies on the isolated rat heart perfused at constant rate (10 ml per minute) with 5% albumin in KRB were designed to investigate the influence of increased extracellular H+ ion on the pacemaker and contractile performance and the anaerobic metabolism of the anoxic heart. During anoxia, (96% N 2 , 4% CO 2 ) at pH 7.4 in the presence of anaerobic substrate (glucose 200 mg/100 ml) the isolated heart performed spontaneously for 30 minutes. During this period there was active anaerobic lactate production (0.84 mM/30 min/gm of protein), whereas the myocardial content of ATP and total adenine nucleotides was significantly reduced when compared to aerobic control levels. Diminution in anoxic perfusate to pH 7.0, accomplished by addition of HCl, resulted in a significant depression of cardiac pacemaker activity and left ventricular contractile performance unassociated with a diminution in myocardial anaerobic lactate production, ATP, or total nucleotide content when compared to pH 7.4. During anoxia at pH 6.6 there was a marked decrement in cardiac pacemaker activity and ventricular performance, associated with slight diminution in anaerobic lactate production, ATP, and total nucleotide content of the myocardium when compared to pH 7.4. The data demonstrate that the decrement in function of the anoxic heart imposed by increasing extracellular hydrogen ion concentration to pH 7.0 does not primarily involve suppression of glycolytic activity. Only at pH 6.6, when cardiac performance is markedly suppressed, could evidence for diminished anaerobic glycolysis be elicited. These observations are consistent with the thesis that increased hydrogen ion concentration dominantly affects the energy utilization phase of cardiac metabolism.