The rheology of non-melt processible polytetrafluoroethylene (PTFE) pastes has been studied using a capillary rheometer. It was found that fibrils of submicron, dimensions are created during PTFE paste processing, which are responsible for the final strength the extrudate. The mechanism of fibrillation is explained in terms of the unwinding of crystallites. To describe the effects of die design, a simple mathematical model has developed. The model takes into account the elastic-plastic (strain hardening) and us nature of the material in its non-melt state. model predictions are found to be consistent with experimental results obtained from macroscopic pressure drop measurements and flow visualization experiments.