Flow experiments using a capillary rheometer with 0.2% xanthan aqueous solutions were carried out to give experimental evidence of the dependence of the slip velocity on the geometrical parameters of the rheometer. The Mooney method was used to evaluate the slip velocityVs. It was found thatVsis an increasing function of the wall shear stress and also of the length to diameter ratioL/Dnot considered in the Mooney theory. That is,Vs=Vs(τw,L/D), although, for each τwvalue,Vsbecomes independent ofL/Dat largeL/D. In addition, a method to determine the excess pressure drop due to end effects is outlined and experimental results presented. The differences between this method and the one by Bagley are highlighted.