Blood flow in the left coronary artery is lower in the systole than in the diastole. This difference is attenuated in the presence of severe stenosis, which affects the flow more during the diastole than during the systole. Some explanations have been suggested: epicardial vasodilatation distal to the stenosis, a decrease in myocardial contractility and impairment of the intramyocardial pump effect. The present investigation in anaesthetized dogs showed that, in the presence of severe stenosis, the attenuation of the diastolic-systolic coronary flow differences occurs together with distal vasodilatation in the epicardial layers of the myocardium. This attenuation may be even greater if further vasodilatation is induced by increasing the heart rate. Mo evidence of reduced myocardial contractility was observed. In addition, it was found that the onset of the systolic rise of the coronary blood pressure below the stenosis occurs before that of the aortic blood pressure. This finding may serves as evidence for the role played by the intramyocardial pump mechanism in causing the systolic reduction of coronary flow. Since this mechanism is believed to propel some blood back into the aorta during the systole, the impairment of this retrograde flow caused by the stenosis could also account for the reduction of the diastolic-systolic flow differences.