Abstract - A constitutive rheological equation is proposed for the paste extrusion of  polytetrafluoroethylene (PTFE) that takes into account the continuous change of the  microstructure during flow, through fibril formation. The mechanism of fibrillation is  captured through a microscopic model for a structural parameter,ξ. This model  essentially represents a balance of fibrillated and unfibrillated domains in the PTFE paste  through a first order kinetic differential equation. The rate of fibril formation is assumed  to be a function of the strain rate and a flow type parameter, which describes the relative  strength of straining and rotation in mixed type flows. The proposed constitutive equation  consists of a shear-thinning and a shear-thickening terms, the relative contribution of the  two being a function of ξ. Finite element simulations using the proposed constitutive  relation predict accurately the variations of the extrusion pressure with the apparent shear  rate and die geometrical parameters.