The aim of this study was to support the hypothesis that aortic distensibility, resulting in a Windkessel effect of the high pressure compartment, may influence coronary blood flow, principally in the subendocardial layers. A hydraulic model of the arterial circulation including a branch representing the coronary circulation, was constructed. A pump giving pulsed flow with ejection parameters and cycle adjusted so as to produce almost physiological pressure curves, was used. Measurements of "coronary" flow were made with aortic models of the same dimension but of different compliance. The output of the ventricular pump and the hydraulic resistance of the model being constant, considerable variations in coronary flow were observed. With the most rigid aorta (compliance: 0,005 cm3/mm Hg) coronary flow was only 1% of the output of the ventricular pump. With a more distensible aorta (compliance: 0,6 cm3/mm Hg) coronary flow rose to 18% of the pump's output. These results were obtained under conditions very different from those present in vivo: however, they do raise the question of whether an alteration in the mechanical properties of the aorta due to ageing or disease can have an appreciable influence on coronary blood flow.