The effects of aging on pressure perturbations during a start-up flow were experimentally investigated on a polymer suspension flowing through a pipe, which demonstrated a prominent undershooting of the pressure difference during the start-up flow of sufficiently aged fluids. A mechanism is proposed to explain this phenomenon via aging-dependent softening of the fluid. This mechanism is supported by rheological measurements showing that changes in rheological characteristics are associated with aging and shear history. (C) 2015 Elsevier B.V. All rights reserved.