Modeling of the response of solidlike polymers is often difficult, not only due to the highly nonlinear behavior of the materials but also because of the difficulty of obtaining relevant material data in the laboratory. Here, we examine the possibility of using concepts from finite elasticity theory to describe the isochronal single-step stress relaxation response for a polymer glass (polycarbonate) far below its glass transition. Torque and normal force measurements from torsional stress relaxation experiments are used to obtain isochoric values for the derivativesW1andW2of the strain energy density function in terms of the deformation invariants at specific time values (isochrones). The values ofW1andW2are then used to determine isochronal values of the Valanis–Landel [Valanis, K. C. and R. F. Landel “The Strain–Energy Function of a Hyperelastic Material in Terms of the Extension Ratios,” J. Appl. Phys.38, 2997–3002 (1967).] (VL) function derivativesw′(λ)and to predict the tension and compression responses for different deformations λ below yield. It is found that, for the conditions examined, the experimentally obtained tension and compression responses are well described within the VL framework, despite the fact that polycarbonate is a compressible material. This success suggests that the set of experiments required to describe the nonlinear behavior of glassy materials may be smaller than previously thought. Also, volumetric measurements in the uniaxial deformations indicate a densification of the glass at large deformations and long relaxation times, which is consistent with concepts in the literature that invoke mechanically accelerated aging to describe mechanical and structural interactions in the physical aging of glassy polymers.