Series of isothermal gas-displacement of a polystyrene melt in a circular cylinder were performed. The experiments show an increase in the steady fractional coverage above a Newtonian level (m = 0.6) at very low Deborah numbers, where the melt behaves like a diluted un-entangled system. At higher Deborah numbers, where the melt behaves as an entangled melt system, the steady fractional coverage decreases. Numerical flow calculations of the time dependent axisymmetric displacement in a circular cylinder were performed using the Lagrangian integral method. A molecular stress function (MSF) constitutive model was used to describe the fluid. The finite element simulations show quantitative agreement within experimental error for Deborah numbers 0.3 < De < 1, based on a MSF model without molecular extension corresponding to a Doi-Edwards strain. At higher Deborah numbers molecular extension needs to be included in the MSF model to obtain the same agreement. (c) 2007 Published by Elsevier B.V.