The influence of a change in aortic compliance on the systolic/diastolic filling of the coronary vascular system was investigated in a canine heart-lung preparation. Changes in compliance were produced by opening and closing a Windkessel attached to the ascending aorta and a reservoir system. The percentage of total coronary flow occurring during systolic ejection increased significantly with a decrease in aortic compliance, whereas isovolumetric and diastolic coronary flow decreased. These effects were also present when cardiac work was changed by altering either afterload or bypass flow. The hypothesis was also tested in whole animal experiments in which left circumflex coronary flow was divided into systolic and diastolic components while the ascending aorta was partially constricted. A similar increase in the proportion of systolic flow was seen even at the same total circumflex flow, cardiac output, heart rate, and mean aortic pressure levels. It is suggested that this finding may have important implications in human disease.