1. Studies have been made on the isolated blood-perfused heart of dogs in which isometric tension development at various settings of resting tension (RT) was measured at various levels of O(2) delivery rates controlled by altering (a) coronary blood flow (CBF), (b) O(2) capacity or (c) O(2) saturation of the perfusate. Measurements were also made of O(2) consumption and vascular perfusion resistance.2. The capacity of the left ventricle to develop tension at any given setting of resting tension was found to be directly correlated with changes in O(2) delivery rate using any of the above three methods of altering the latter.3. The slopes of the curves relating resting tension to developed tension are positively correlated with total O(2) delivery to the heart.4. The O(2)-dependent metabolic effect upon tension production was found to be slow in development, in contrast to the Frank-Starling effect, which reached full development in the first heart beat after a change in resting length and tension.5. The O(2) consumption of the isometrically contracting heart is strongly correlated with the O(2) delivery rate at all particular values of resting tension and related developed tension.6. The metabolic state of the myocardium as determined by the rate of O(2) delivery within physiological ranges is (a) a direct major determinant of the tension-producing capacity of the heart muscle and (b) determines the magnitude of adaptation via the Frank-Starling mechanism.