From constant strain-rate experiments in extensional flows, nonlinear strain measures and effective damping functions are derived for a linear high-density polyethylene melt. Experimental results are compared with predictions of two molecular theories, the Doi-Edwards model and the molecular stress function approach of Wagner and Schaeffer. Orientation (but not stretch) of macromolecules at small and large strains is well represented by the Doi-Edwards orientation tenser. Stretch of macromolecules is found to be isotropic as predicted by the molecular stress function model, which is in excellent agreement with the data. For the latter model, a novel strain energy function is presented.