An attempt was made to differentiate between autoregulatory coronary vasodilation and changes in vasomotor tone produced by factors extrinsic to the heart. this was done by investigating the relation between local cardiac force and local coronary blood supply. Intracellular NADH redox levels were also measured in order to further elucidate the oxygen balance under various experimental conditions. In anaesthetized open-chest dogs, local blood supply was estimated with the aid of a thermistor probe, and the oxidation-reduction state of mitochondrial pyridine nucleotide was measured by a surface fluorometric technique. Local myocardial contractile force, as well as blood pressure and ECG were recorded simultaneously with the above parameters. The heart was paced at frequencies from 60/min to 300/min with an electronic stimulator, under both normoxic and hypopneic conditions. It was found that elevation of heart rate caused a progressive increase in local blood flow during both normal and hypopneic ventilation. The absolute flow values during hypopnea were approximately double those during normoxia. Heart rates above 120/min or 150/min resulted in a progressive increase in NADH fluorescence. This response to elevated heart rate was less prominent or absent during hypopnea. Contractile force during hypopnea was greater at elevated heart rates than during normal breathing. Data are brought which suggest that whereas vasodilation following increased heart rate is probably due to an autoregulatory mechanism, the marked vasodilatatory effect of hypopnea is related to elevated arterial CO2 levels. It is suggested that hypercapnia markedly stimulates extrinsic coronary vasodilation thereby supplying enough oxygen to maintain contractility even at very high heart rates. Moreover, intracellular O2 concentration (mitochondrial NADH level) is maintained at a normal level despite the greatly increased demand.