  We here present two simplified models aimed at describing the long-term,
irregular behaviours observed in the rheological response of certain complex
fluids, such as periodic oscillations or chaotic-like variations. Both models
exploit the idea of having a (non-linear) rheological equation, controlling the
temporal evolution of the stress, where one of the participating variables (a
"structural" variable) is subject to a distinct dynamics with a different
relaxation time. The coupling between the two dynamics is a source of
instability.
