To determine whether conduit artery size affects functional responses, we compared the magnitude, time-course and eliciting shear rate stimulus for flow-mediated dilatation (FMD) in healthy men (n=20, 31±7 yrs). Upper limb (brachial and radial) and lower limb (common and superficial femoral) FMD responses were simultaneously assessed, whilst popliteal responses were measured in the same subjects during a separate visit. Glyceryl trinitrate (GTN)-mediated responses were similarly examined. Edge-detection and wall-tracking of high resolution B-mode arterial ultrasound images, combined with synchronized Doppler waveform envelope analysis, was used to calculate conduit artery diameter, blood flow and shear rate continuously across the cardiac cycle. Baseline artery size correlated inversely with the FMD response (r=-0.57, p<0.001). Within-artery comparisons revealed a significant inverse correlation between artery size and FMD for the radial (r=-0.66, p=0.001), brachial (r=-0.55, p=0.01) and popliteal artery (r=-0.48, p=0.03), but not the superficial and common femoral artery. Normalization of FMD responses for differences in eliciting shear rate did not abolish the between-artery relationship for artery function and size (r=-0.48, p<0.001), suggesting that differences between artery function responses were not entirely due to size-related differences in shear rate. This was reinforced by a significant between-artery correlation for GTN responses and baseline artery size (r=-0.74, p<0.001). In summary, systematic differences exist in vascular function responses of conduit arteries which differ in size. This raises the possibility that differences in artery size within or between individuals may influence functional responses.