In vitro experiments have shown that the shear stress exerted by flowing blood on the endothelial surface affects the morphology of the vascular wall and the release of vasoactive substances and growth factors by that wall. It is believed that the caliber of a vessel adjusts to the local shear stress to maintain a specific value of the shear stress. The local shear stress follows from local shear rate by multiplying shear rate by the local blood viscosity. The present article describes a method in which ultrasound techniques are used to assess transcutaneously the time-dependent wall shear rate in vivo in arteries. This method is applied to the assessment of wall shear rate in the common carotid artery of volunteers, presumed to be healthy, in two age categories (young age group, 20 to 30 years old, n = 8; old age group, 60 to 70 years old, n = 6). Although the peak shear rate in the young age group is markedly higher than in the old age group, the mean shear rate averaged over a cardiac cycle has the same value of 210 s-1 for both groups, corroborating earlier observations that mean shear rate and, hence, mean shear stress are maintained at a particular value. Conversion of the observed shear rates to shear stresses, assuming a blood viscosity of 3.5 mPa.s for both age groups, gives shear stresses of approximately 0.7 Pa. This is a factor of two lower than the shear stresses estimated from the relation between volume flow and artery caliber (1.5 Pa).