The transient birefringence and the transient tensile stress of a polystyrene melt at 140 degrees C elongated with constant stretch rate (lambda) and constant Hencky strain rate (epsilon) are presented. Compared with the response during linear viscoelastic deformation, the values of the reduced birefringence and the reduced stress are higher when the elongation is performed with epsilon = const., and are lower when the elongation is carried out with lambda = const.. The stress-optical coefficient turns out to be a function of duration and rate of deformation.