The present study was planned to investigate whether or not, after complete suppression of vasomotor tone, increases in intravascular blood pressure distend the coronary vasculature causing passive decreases in the resistance to the coronary arterial inflow during the diastole. In anaesthetized dogs, aortic and left ventricular pressures and flow in the left circumflex coronary artery were recorded. Coronary flow was derived using an electromagnetic flowmeter. Transient (10 s) increases in intravascular blood pressure in a range above 70 mm Hg were produced by mechanical constriction of the descending thoracic aorta. In the presence of a normal vasomotor tone the increase in blood pressure caused an autoregulatory increase in the mean diastolic coronary inflow resistance. After maximal vasodilatation by dipyridamole, no change in inflow resistance was induced by the increase in intravascular blood pressure. It may be argued that while a non-maximal vasodilatation is reported to increase coronary distensibility, at a blood pressure of 70 mm Hg the complete suppression of the vasomotor tone brings the vascular radius to a size which cannot be further distended by an increase in blood pressure.