The dynamics of retrograde coronary flow in aortic valvular stenosis was investigated in an in-vitro pulse duplicating system which had the capability of simulating coronary flow. The ventricular chamber of the pulse duplicator consisted of an opaque elastic sac molded from rubber in the shape of a left ventricle. The aortic test section consisted of an acrylic mold of the root of the aorta of a calf, which included the sinuses of Valsalva and the entrance region of both the left and right coronary arteries. Flow in the left coronary artery was modeled to deliver both a systolic and a diastolic component of flow. Studies were performed with normal porcine valves in the aortic and mitral positions and were repeated with a human stenotic valve in the aortic position. Pressures were measured in the aorta, left ventricle, and at the ostium of the left coronary artery with catheter-tip micromanometers. In the presence of a normal aortic valve, total coronary flow was adjusted to 120 ml/min of which 21 percent of the flow occurred during systole. The phasic pattern of coronary flow was similar to that shown in vivo. In the presence of a stenotic aortic valve, a small amount of retrograde coronary flow (<1 percent of total coronary flow) was observed; and this occurred during the initial phase of systole. Retrograde coronary flow during systole appears to have resulted from compression of the collapsible segment of the simulated coronary artery. This was caused by the elevated simulated intramural pressure.