It is shown that the biaxial drawing of poly(ethylene terephthalate) at temperatures above the glass transition temperature T-g can be satisfactorily described by the Ball model of rubber elasticity (Polymer, 1981, 22, 1010), providing that the chain entanglement density is assumed to be a linear function of the logarithmic shear strain rate. With this assumption the behaviour for uniaxial, constant width and equibiaxial drawing can be fitted by a single set of model parameters. The validity of the network model has been confirmed by photoelastic studies, provided that the draw ratios do not reach a level where strain crystallization occurs to any significant extent.