A physically based constitutive law for the deformation of polymers is applied to the stretching of polypropylene to large deformations at elevated temperatures. In this deformation regime, which is applicable to many forming processes, necking of the material is a persistent feature. The theory is elastic in nature, but includes the necking phenomenon as an inherent property. It is incorporated into a commercial finite element code and used to model a number of different experimental modes of deformation, both uniaxial and biaxial. Comparison is made with the experiments and it is found that both strains and forces are represented realistically, even though the true nature of the material is viscoelastic. Some of the discrepancies in the model predictions are traceable to its elastic nature.