Pressure-volume relationships and myocardial oxygen consumption (V˙O2) were obtnined in an isolated heart during ventricular fibrillation, potassium-induced arrest, and isovolumetric contractions. The heart was found to be less distensible in fibrillation than in diastole or arrest. The V˙O2increased from 4.6 to 14.6 cc./100 Gm. LV/min. as the fibrillating ventricle was distended. During arrest the V˙O2remained constant despite ventricular distention. When the fibrillating ventricle was distended beyond a critical point coronary flow was compromised and the V˙O2decreased. Beyond and only beyond this point could the V˙O2be increased by the greater coronary flow induced by a higher perfusion pressure.