Abstract: In the present paper a concept of "quasi-short memory" is introduced instead of "short memory" in order to develop a theory for disturbed constitutive equation for the basic flow of non-Newtonian fluids. The constitutive equation is derived for general basic flow and rheological models. The constitutive equation of second order for disturbed extensional flow is used to study the instability of stretching sheet of viscoelastic fluid. The instability criterion is given for general rheological model and the ap...