Conventional tensile testing applied to high density polyethylene can lead to erroneous impressions of the tensile response of the material due to a local reduction in cross section of the sample. Several workers have developed novel tensile testing techniques to measure the response of a small element as it deforms. The true stress true strain curve that results describes tensile deformation of the material in a geometry-independent manner. Here, results from previous workers, together with some of our own, are interpreted in terms of the Haward-Thackray spring-dashpot model, in which the spring defines a strain hardening process according to the theories of high elasticity and the dashpot describes a strain-independent viscous process. The effects that temperature, strain rate, and molecular mass have on each process are investigated. For a pipe-grade, modified high density ethylene copolymer, sufficient data have been measured to interpret the effects of strain rate and temperature in accordance with an Eyring flow process, where the parameters for the two mechanisms are found to be similar.