The purpose of this study was to determine the influence of radial motion of an arterial wall on the shear stress that flowing blood imposes on the wall (wall shear stress). Wall shear stress is known to influence endothelial cell function and may play a role in atherogenesis, but its magnitude and distribution in the circulation are not well understood. To simulate arterial wall motion, we used both straight and curved rubber tubing models in a mock circulatory system incorporating a variable impedance element distal to the model aorta. Wall shear rate was measured with a hot-film anemometer probe mounted flush on the tubing wall and free to move with the wall. Wall shear stress was determined as the product of wall shear rate and viscosity. By changing the distal impedance element, we determined the influence of the (temporal) phase angle between pressure and flow, or equivalently, tube diameter and flow, on wall shear stress. We observed a fivefold increase in peak wall shear stress and the onset of intense wall shear stress reversal sinusoidal flows as the phase angle between pressure and flow was reduced from -60 degrees to -80 degrees. Wall shear stresses were insensitive to this same phase angle when its value was between -10 degrees and -60 degrees. Theoretical predictions, also presented here, are in accord with these observations. The phenomena we have observed in elastic tubes may be important in the arterial system because the phase angle between the first harmonic of pressure and flow in the aorta of humans is usually near our sensitive range. This same phase angle becomes more negative in hypertensive patients and is influenced by vasoactive drugs.