Previous work was extended to the study of the time dependence of the stress-strain behavior of rubberlike polymers. That dependence is conveniently described with the help of the so-called strain energy density function, W. The knowledge of W is crucial for the determination of the state of stress at any given point in a deformed body. The model results indicate that the partial derivatives of W with respect to the strain invariants, I 1 and I 2 , are strongly dependent on time. It is shown that their sum, however, should be constant as a consequence of the Valanis-Landel hypothesis