The reactive hyperemia (RH) test commonly used to assess flow mediated dilation (FMD) in human conduit arteries creates a large, transient shear stimulus. PURPOSE: To identify the relative importance of the peak vs. the duration of shear rate in determining the peak FMD response. METHODS: Brachial artery (BA) blood flow velocity was measured with Doppler ultrasound and echo ultrasound BA images were analyzed with automated edge detection software (shear rate = velocity/diameter). 1) Duration manipulation experiment (DME) - 10 healthy males underwent six RH trials with different hyperemic durations (10s–2 min). The duration of hyperemia was manipulated by re-inflating the occlusion cuff after 10s, 20s, 30s, 40s, or 50s. 2) Peak manipulation experiment (PME) - 8 healthy males underwent three RH trials. Three peak shear rate magnitudes (large (L), medium (M) and small (S)) were created by applying BA compression upon cuff release. The post-peak area under the curve (AUC) of shear rate was also controlled with arterial compression. RESULTS: DME - Peak shear rate (AUC in 1st 9s post cuff release) was not different between trials (P = 0.309). Re-inflation of the occlusion cuff at times indicated resulted in the remaining AUC being significantly different between trials (10 s −2025.37 ±504.21; 20 s −3393.54±857.03; 30s-4329.30±1198.47; 40s–4784.20±1345.00; 50s–5215.83±1466.63 2min −7124.39±2160.67 p<0.001). The respective peak % changes in diameter were: 10s-2.67±1.34; 20s-6.21±1.95; 30s-7.89±2.95; 40s-8.3±3.21; 50s-7.91±3.20; 2min-9.34±4.10, with 10 and 20 s being significantly less than 2min (p<0.001). PME - Peak shear rate (AUC in 1st 9s post cuff release) was significantly different between trials (L- 998.04±305.16, M-700.66±225.89, S-487.48±1 67.55 p<0.001). The AUC of the post-peak shear rate was not significantly different between trials (P = 0.695). The peak % change in diameter was not significantly different between trials (L-6.99±2.72%, M-7.56±2.6%, S-6.87±1.99%, P = 0.437). CONCLUSIONS: Manipulation of the duration of the shear rate had an impact on the peak FMD response while independent manipulation of peak shear rate did not. We conclude that the shear rate AUC following the peak is a critical determinant of the peak FMD response and needs to be accounted for in quantifying the stimulus for FMD. Supported by ACSM Doctoral Research Grant, NSERC, CFI New Opportunities Fund