Extrusion tests were performed by forcing a well-characterized model yield stress fluid from a cylindrical cartridge through various cylindrical extrusion dies using a variety of different piston velocities. In this study the Bingham number within the die ranged from 0.1 to 10. MRI techniques allowed for the non-invasive determination of the local velocity within the extruded material in the range [0.015; 20 mms(-1)]. The velocity profile within a very long die was determined by MRI and agreed very well with the analytical results for the flow of a Herschel-Bulkley fluid within a conduit using parameters determined from independent rheometrical tests, validating both the rheological approach and the accuracy of the MRI techniques. Although the velocity was determined by MRI in the upper and lower zones separately, the intersection of these zones showed great agreement, providing an entire view of the extrusion process. In the range of Bingham number studied, the velocity field for a given contraction ratio appeared similar when scaled by the piston velocity, with a dimpled acceleration zone above the die and lateral dead zones varying negligibly with the piston velocity. For a further analysis the experimental results were compared with the results of numerical simulations. Finite element simulations using an elastic solids model were performed to provide this comparison. It was found that this model did well in representing the characteristics of extrusion flow seen in the experiments; an aspect that was not present in the biviscous simulations. The MRI results show that for the range of values studied, both the piston velocity and the contraction ratio have little effect on the characteristics of the flow, including the size and location of the apparent dead zones. It was found that with an appropriate scaling the central, longitudinal velocity follows a master curve. A decreasing contraction ratio, on the other hand, appears to increase the size of the weak velocity region, in contrast with the simulation results. (C) 2010 Elsevier B.V. All rights reserved.