OBJECTIVE The aim was to study the effects of altered heart rate and vasoactive drugs on the blood velocity patterns in the region of an arterial bifurcation. METHODS Blood velocity profiles were measured in an exposed iliofemoral bifurcation of paced dogs using a pulsed Doppler ultrasound velocimeter with high temporal and spatial resolution. RESULTS Decrease of the heart rate from 120 beats.min-1 (2 Hz) to 60 beats.min-1 (1 Hz) increased the peak forward velocity (30%), the peak reverse velocity (20%), and the duration of reverse flow (25%). Each drug caused qualitatively similar changes in velocity patterns at both heart rates. The systemic administration of angiotensin II reduced peak forward velocity (-26% at 2 Hz and -33% at 1 Hz) and forward flow duration (-15% at 1 Hz), the peak reverse velocity (-30% at 1 Hz), and reverse flow duration (-20% at 2 Hz and -28% at 1 Hz). Glyceryl trinitrate also reduced the peak forward velocity (-19% at both 2 and 1 Hz) but prolonged forward flow duration (28% at 2 Hz and 17% at 1 Hz) and that of reverse flow (45% at 2 Hz and 24% at 1 Hz), and also decreased the degree of oscillation (-16% at 2 Hz). Barnidipine hydrochloride (a calcium channel antagonist) also increased the duration of forward flow (48% at 1 Hz) and of reverse flow (31% at 2 Hz) but reduced the peak reverse velocity (-29% at 1 Hz) and flow oscillation (-22% at 2 Hz and 20% at 1 Hz). CONCLUSIONS These dramatic changes in the pattern of blood flow, including alterations in the amplitudes and durations of the different phases of the flow cycle, are expected to have important consequences on the shear dependent responses of endothelial cells in the region of the bifurcation.