The development of orientation in molten narrow molar mass fractions of polystyrene during shear flow in the non-Newtonian shea. rate range (11-24 s(-1)) at temperatures between 225 and 275 degrees C has been assessed by measurement of the infrared dichroism. Deviation from linearity between shear stress and orientation was found at high shear stresses in the steady state region. A sudden change in the shear rate caused a change in the shear stress that was one to two orders of magnitude faster than the response in the Hermans orientation function (f). The steady-state level of the Hermans orientation function (f(ss)) was dependent on temperature, shear rate and molar mass (M) : f(ss) = C-1-C-2/M (where C-1 and C-2 are constants). The time to reach steady state on the inception of shea. was proportional to M-1/2. The recovery of the isotropic state after the cessation of shear followed initially a simple exponential law : f proportional to e(-1), where t is the time and tau is the relaxation time. The relaxation time showed only a weak molar mass dependence (tau proportional to M-0.8), indicating that the decay of orientation was initially governed by processes other than reptation.