This study examines the effect of porous media of different filtering rates placed at the entrance of an extrusion die, on the processability of polymer melts, both in the absence and presence of wall slip. Two fluids were considered: a polydimethylsiloxane (PDMS) oil and a high density polyethylene (HDPE) which can exhibit slip at the wall when extruded. The total pressure drop can be greatly increased when a porous medium is introduced. This increase is a smoothly growing function of the rate of flow Then, flow curves do not show anymore discontinuity or hysteresis. As a result, porous media considered in this study affect oscillation regimes, which may appear when a simple die is used. Thus, the transition from stable flow regimes with adherence to flow regimes with wall slip takes place progressively. As for the influence of porous media on the triggering and the aspect of melt fracture type instability, photographs of the extruded rod taken right at the die exit show that this phenomenon is clearly mitigated since its frequency is higher and its amplitude smaller than what they are when no porous medium is used. Moreover, the use of a porous medium with a high filtering rate significantly delays the occurrence of melt fracture. It is associated with a smooth and transparent extrudate, free from any die swell. It is possible to relate the observed effects on upstream instability, considering chains disentanglement through the porous medium. Moreover, when flowing through the porous medium the polymer is pre-sheared before entering the extrusion die, which leads to the reduction of elongation strains associated with the onset of instabilities. (C) 2000 Elsevier Science B.V. All rights reserved.