Stress relaxation behavior of concentrated solutions of high molecular weight polystyrene following a step shear strain is studied using both mechanical and optical birefringence techniques. Using the stress‐optic law, which we find to be valid for our solutions, we obtain time‐dependent shear stress and first normal stress difference values from birefringencemeasurement that are free of transducer compliance effects. Similar to the previously reported experimental observations of Fukuda, Osaki and Kurata, we obtain unusually low values for nonlinear shear moduli, much lower than the predictions of Doi‐Edwards model, for the sample with more than about 60 entanglements per molecule. Moreover, the shear stress and first normal stress difference data measured on this sample do not conform to the Lodge‐Meissner relationship, especially at long times, suggesting the formation of regions in which the imposed strain is not homogeneously distributed.