Polytetrafluoroethylene (PTFE) is known to be a polymer that shows inherent microstructure formation during cold processing such as paste extrusion. To model such a complex flow, a viscoelastic constitutive equation is proposed that takes into account the continuous change of the paste microstructure during flow, through fibril formation. The mechanism of fibrillation is captured through a microscopic model for a structural parameter that represents the percentage of fibrillated domains of the paste. The proposed viscoelastic constitutive equation consists of a viscous shear-thinning term (Carreau model) and an elastic term (modified Mooney-Rivlin model), the relative contribution of the two depending on. The viscous and elastic parameters of the model are determined by using shear and extensional rheometry on the paste. Finite element simulations based on the proposed constitutive relation with the measured model parameters predict reasonably well the variations of the extrusion pressure with the apparent shear rate and the die geometrical characteristics. (c) 2007 Elsevier B.V. All rights reserved.