Utilizing the Doppler ultrasonic blood velocity probe, superior mesenteric artery blood velocity (SMABV) was studied in 20 subjects during cardiac arrhythmias. Normal SMABV is phasic and continuous throughout the cardiac cycle, with a substantial diastolic fraction. Atrial, junctional, and ventricular premature beats result in diminished SMABV, dependent on their coupling intervals. Atrial tachycardia, atrial fibrillation, and ventricular tachycardia can cause a marked reduction of SMABV. Termination of tachyarrhythmia is associated with an increased systolic and diastolic SMABV. It is concluded that cardiac arrhythmias can diminish SMABV, providing a basis for some cases of nonocclusive mesenteric ischemia and infarction. The Doppler blood velocity probe system is a useful technique for characterizing SMABV under normal conditions and during cardiac arrhythmias.