Utilizing a Doppler ultrasonic flowmeter catheter, phasic instantaneous aortocoronary saphenous vein bypass blood velocity was recorded during various atrial arrhythmias in 17 patients. Atrial premature beats, supraventricular tachycardia and atrial fibrillation reduced peak bypass graft blood velocity in relation to the heart rate and diastolic cycle length. Such changes were more evident during the systolic fraction of bypass blood flow. In a single subject, systolic blood velocity alternans was observed during the course of atrial tachycardia. It is concluded that such study provides insight into the dynamics of bypass graft blood flow velocity during atrial arrhythmias.