Continuous simultaneous measurements of coronary venous oxygen saturation and coronary venous blood flow were performed in 10 open chest dogs and 13 conscious human patients, using a combination of fiberoptic oximetry and thermodilution. Measurements were made during transient coronary arterial occlusion and during the intracoronary administration of nitroglycerin or isoproterenol. Two patterns of coronary vasodilation were distinguished: in primary (demand-independent) vasodilation, both coronary flow and coronary venous oxygen saturation increased, while myocardial oxygen consumption remained constant. In secondary (autoregulatory) vasodilation, coronary flow increased as coronary venous oxygen saturation decreased, corresponding to an increase in myocardial oxygen consumption. These observations could not have been made if either coronary venous blood flow or oxygen saturation had been measured alone; thus, this investigation demonstrates the utility of this combination of techniques in the study of the coronary circulation.