During a study of biaxial hot-drawing of amorphous poly(ethylene terephthalate), the yield stress was determined in the temperature window between the glass transition and the rubber-like plateau, for various Values of the strain-rate ratio, with one nominal strain-rate kept constant at 1 s(-1). Results were used to test predictions of the constitutive model proposed earlier (Buckley, C. P. and Jones, D. C., Polymer 1995, 36, 3301), which expresses flow in terms of a three-dimensional generalisation of the Eyring rate process. Excellent agreement was obtained for the variation of yield stress with strain-rate ratio.