Almost all basic work in rheology, both experimental and theoretical, has dealt with isothermal flows, with the temperature being held constant during any one measurement. The present paper presents systematic studies of the nonisothermal behavior of two polystyrenes (a commercial sample and a sample of sharp molecular weight distribution). The experiments involve stress growth and stress relaxation during elongation in an Instron tensile tester, with rapid changes of temperature being imposed during the pulling and after it. Generalized non‐isothermal viscoelastic theory is proposed and tested. The generalized time‐temperature superposition involved there works well in simple stress relaxation (when the temperature changes are imposed after the pulling) but in the simultaneous type of experiments (when the temperature changes are imposed during the pulling), the accommodation to the new state seems to be more rapid than expected from theory.