The objective of this work is to represent general nonlinear viscoelasticity by a model based on a modified two‐network theory. As a basic problem we examine Neubert and Saunders' data (1958) on the permanent set of crosslinked natural rubber samples after heating in a state of pure shear or simple extension. It is evident that the preferred configuration changes with time of heating. Our approach is to associate the preferred configuration with an internal measure of length in an initially isotropic material which becomes anisotropic after heating in a deformed state. Using Ericksen and Rivlin's work (1954) on anisotropic materials and a strain energy function which reduces for isotropic materials to Rivlin and Saunders' function (1951), we show an excellent consistency with the Neubert and Saunders' data. We also show consistency with some more recent data due to Djiauw and Gent (1973).