The application of the steady-state assumption to the enzyme species in kinetic models of biochemical systems is described. A set of rules for the derivation of the rate laws of complex enzymes under this assumption is available (King & Altman, 1956). Two programmes, which carry out a symbolic derivation of this kind, and can be used as part of a generator system for simulation by this method, are described in detail. The limitations and advantages of this approach are discussed.