Rheological tests based on steady‐shear and transient experiments are used to discriminate between three classes of integral constitutive equations: the K‐BKZ, the LeRoy‐Pierrard, and the Carreau equations. The tests derived from the given equations require no prior evaluation of model parameters. Reliable data obtained for steady‐shear, stress growth, and stress‐relaxation experiments conducted with various polystyrene solutions are used. The Carreau and K‐BKZ equations appear to be the most flexible, but none of the equations is capable of describing the whole spectrum of behavior observed in relaxation. The strains at which the maximum values of the shear and normal stresses are observed for the polystyrene solutions increase with the applied shear rate. This behavior can be described by the Carreau equation. This contrasts with results reported in the literature on low density polyethylene for which the strains were found to be constant.