A new ultrasonographic machine (FRP II) has been developed to measure vessel wall shear stress. A multigate ultrasound probe sends an ultrasound beam simultaneously focused in subsequent points 0.2 mm from each other along the transverse axis of a blood vessel. Blood velocity is measured by cross-correlation technique, which allows a rapid and economical analysis. Thus, the instantaneous (every 5 msec) blood velocity profile is reconstructed for the duration of the entire cardiac cycle. In order to verify the precision and sensitivity of the FRP II in measuring shear stress, 36 measurements were performed on the common carotid artery in 9 hypertensive subjects in different hemodynamic conditions. The FRP II-measured shear stress (the product of the shear rate and blood viscosity) was compared to that calculated by the Womersley's mathematical model (Y = 2K.Vcl/D, where Y = shear rate, Vcl = vessel center line blood velocity, D = vessel diameter). A good correlation (r = 0.77, p < 0.0001) was found between the peak systolic shear stresses measured by FRP II and that calculated by the Womersley's mathematical model, although an underestimation for higher values was observed with the latter method. In conclusion, we propose a new ultrasonographic instrument to measure "in vivo" the vessel wall shear stress.