Capillary extrusion flow curves of four anionic and five polydisperse polystyrenes have been measured at five temperatures between 170 and 237°C. The non-Newtonian flow curves of the narrow distribution polymers can be represented by a common master curve in terms of the viscosity ratio (η/η0) and the product of shear rate and a relaxation time. The polydisperse polymers do not yield a single flow curve. Current molecular and semiempirical models do not yield master curves which fit the experimental plots. Exceptions are Pao's model for monodisperse polymers and an empirical relation presented in this article which fits the data of the five commercial polymers studied. The existence of a general theory which represents experimental polymer flow curves satisfactorily has yet to be demonstrated. An acceptable model should predict rheological parameters correctly and should also show why experimental data for polymer melts scatter rather severely. Such scatter often reflects lack of control of an important experimental parameter and it is suggested that this parameter may be connected with differences in the physical significance of various estimates of zero shear viscosity, η0. Shear history of the samples may also be an important factor.