The rheological and processing behavior of BaSO4-filled medical-grade (additive-free) thermoplastic polyurethanes (TPUs) was investigated. The rheology and the single-screw and twin-screw extrusion processing of filled TPU were found to be complicated by the moisture and the air entrained into the suspension upon the incorporation of the filler BaSO4. It was observed that the 20 vol% BaSO4-filled TPU exhibits decreased, rather than increased, shear viscosity and elasticity (as manifested through smaller storage modulus values in small-amplitude oscillatory shear flow) in comparison to unfilled TPU in the temperature range of 190degreesC to 200degreesC at which these materials are usually processed. The moisture remaining in the BaSO4 will hydrolyze the polymer, leading to a decrease in the molecular weight of the TPU. However, the moisture contained in the filler itself is not sufficient to explain the significant reductions in viscosity and elasticity of the suspension upon the compounding of the BaSO4. It is shown that the major factor giving rise to the reductions in elasticity and the viscosity of the suspension of TPU and BaSO4 is the air (and the moisture air contains) entrained into the extruder during the feeding of the BaSO4. Air, carried with the filler into the extruder, is entrained into the suspension to hydrolyze the TPU and to further impart a foamy structure under typical processing conditions to significantly reduce the shear viscosity and the elasticity of the suspension in comparison to unfilled TPU. (C) 2004 Society of Plastics Engineers.