The purpose of this study was to determine the dynamic characteristics of brachial artery dilation in response to step increases in shear stress (flow mediated dilation (FMD)). Brachial artery diameter (BAD) and mean blood velocity (MBV) (Doppler ultrasound) was obtained in 15 healthy subjects. Step increases in MBV at two shear stimulus magnitudes were investigated: Large (L; maximal MBV attainable), and Small (S; MBV at 50% of the large step). Increase in shear rate (estimate of shear stress: MBV/BAD) was L:76.8 ±15.6 s^-1 and S:41.4 ±8.7 s^-1. The peak %FMD was L:14.5 ±3.8% and S:5.7 ±2.1%, P<0.001. Both the L (all subjects) and the S step trials (12 of 15 subjects) elicited a bi-phasic diameter response with a fast initial phase (Phase I) followed by a slower final phase. Relative contribution of Phase I to total FMD when two phases occurred was not sensitive to shear rate magnitude (r² = 0.003, slope P= 0.775). Parameters quantifying the dynamics of the FMD response (time delay; TD, time constant; &#964;) were also not sensitive to shear rate magnitude for both phases (Phase I - TD r² = 0.03, slope P=0.376, r² = 0.04, slope P=0.261; Final Phase TD r² = 0.07, slope P=0.169, r² = 0.07, slope P=0.996). These data support the existence of two distinct mechanisms, or sets of mechanisms in the human conduit artery FMD response that are proportionally sensitive to shear stimulus magnitude and whose dynamic response is not sensitive to shear stimulus magnitude.