Computer modelling study of non-stationary kinetics of a three-step reaction mechanism is given. Several types of transient responses, which have different rate-limiting steps in the steady state, are analysed. Non-stationary kinetic behaviour is mechanistically described in detail, using the transient responses of the rates of elementary steps. Suitability of the concept of rate-limiting step in non-stationary kinetics is examined. It is shown that the rate-limiting step concept is not useful for the description of transition state. Nature of the transient behaviour of a set of consecutive reaction steps is revealed.