Stress-relaxation functions in bending and in tension were obtained for several polymer films with widely different microstructures and relaxation characteristics. For all the films tested, non-linear viscoelastic response is observed at strains exceeding 0.8%, with the relaxation time being generally lower the higher the strain. It is also observed that, within the linear viscoelastic range, the relaxation spectra of the bending and tensile moduli are qualitatively different, with the bending modulus relaxing generally more slowly than the tensile modulus. This difference is attributed mainly to a temporal change in Poisson’s ratio, which can be estimated directly from the bending and tensile relaxation functions. Other possible causes for the observed differences in relaxation response are discussed.