Flow curves are based on uniform steady simple shear flow, whether in Couette or Poiseuille realizations. It is still often argued by many authors that, for rheologically simple fluids, a maximum in a flow curve is inadmissible on physical grounds. A simple thixotropic (isotropic viscous) constitutive model, based on a simple structure parameter, which includes yield stress and which is known experimentally to be realistic for gelling suspensions, is shown to predict such maxima. Changes in either stress or flow rate, as in startup, lead to unsteady flows, whose ultimate steady-state nature are history dependent. This history dependence can be removed by introducing a term in the conservation equations to represent diffusion of structure.