Utilizing the Doppler flowmeter catheter, right carotid blood velocity was measured during episodes of catheter-evoked and pacemaker-induced ventricular tachycardias. Such tachyarrhythmias uniformly diminished peak carotid blood velocity, which in the case of catheter-evoked episodes reduced phasic blood velocity by 50%. At driving rates greater than 140 per minute, the average peak carotid blood velocity was 25% to 50% lower during ventricular when compared with atrial pacing. There was a direct correlation between the ventricular pacing rates and the percent decline of peak carotid blood velocity. Such arrhythmia-related carotid blood velocity deficits may account for syncopal episodes in subjects so affected.