The behavior of two polymers, namely, low-density and linear low-density polyethylenes, was studied in transient and steady-state shear and elongational flows. The predictions of Wagner’s model with a damping function using a generalized invariant were calculated. The model appears to be suitable for predictions of the shear and elongational transient flows on the range of strains experimentally tested. The shear flow curves can also be recovered by the model on a very broad range of shear rates. The model is then used to assess extensional data obtained with the convergent flow analysis proposed by Cogswell.