The hypothesis that myocardial contraction attenuates diastolic coronary flow was tested by comparing flow during diastole to flow during a prolonged asystole. The circumflex coronary artery was cannulated and perfused at constant pressure in closed-chest, morphine- and α-chloralose-anesthetized dogs. The heart was paced at 80, 120, 160, or 200 beats/min after atrioventricular heart block under control, intracoronary adenosine, and intravenous norepinephrine treatment conditions. Cessation of pacing while holding coronary pressure constant at the previous diastolic pressure resulted in asystolic circumflex flow that initially equaled the previous diastolic flow during heart rates of 80, 120, and 160 in all treatment groups. Initial asystolic circumflex flow was approximately 5% higher than the previous diastolic flow at a heart rate of 200 beats/min, but this was probably due to an artifact. It is concluded that systolic contraction does not limit diastolic coronary flow at heart rates less than 160 beats/min and probably does not at higher heart rates.