Steady shear flow values of the normal stress ratio PSI = -N2/N1 are compared for monodisperse polystyrene solutions in the concentrated entangled and semidilute entangled regimes. In both regimes, the zero shear rate limiting values are the same : PSI0 = 0.275 +/- 0.005. Surprisingly, the molecular weight required for full entanglement (on the basis of PSI0 scales with concentration as C-1.5+/-0.1, rather than C-1 as commonly supposed. The value of PSI0 is insensitive to fractional free volume and to solvent affinity for polystyrene, even at semidilute concentrations. However, PSI depends on the shear rate, with shear-thinning behaviour in all solvents. Shear thinning of PSI has not been widely reported, probably because of the ’edge fracture’ flow instability in rotational rheometers. In concentrated solutions or melts, edge fracture usually occurs before PSI shear thins by a significant amount.