It is wel-known that changing an enzyme concentration in a metabolic system causes the setting up of a non-steady state and implicitly, the start of a transient state until a new steady-state is established. Nevertheless, since the pioneering works of Kacser & Burns (1973) and Heinrich & Rapoport (1974), most writers on this subject have accepted that the system responds to the perturbation by continuously maintaining the steady state. What are the reasons for the reluctance to deal with transient states? Are there objective aspects, like the lack of appropriate theoretical background to deal with time-dependent control? Are there essential advantages favouring the steadystate treatment over the transient one? Or is it just a matter of subjective conservatism? There are several important questions here, which I intend to deal with in this chapter. It is also aimed at reviewing some basic concepts, bringing forward some less common aspects in the literature and throwing some light where possible. A simple metabolic system will be finally used to illustrate the main ideas of the chapter.
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