The applicability of the pseudo-steady state assumption to an enzyme-substrate complex was examined for an irreversible single-substrate reaction which obeys the Michaelis-Menten mechanism by using computer simulation. The pseudo-steady state solutions of the time courses of the substrate and product were compared with the exact solutions calculated by using a complete set of rate equations under various conditions. The maximum relative errors of the pseudo-steady state solutions to the exact solutions could be correlated well only in terms of a dimensionless parameter, which the authors designate ψ_M and define by the ratio of the pseudo-steady state solution of the intermediate complex at t = 0 to the initial substrate concentration. The simulations showed that if ψ_M ?? 0.05, the pseudo-steady state solutions are accurate within a tolerable error of 5 % and therefore the pseudo-state assumption holds.