A reaction system containing bimolecular subsystems was studied as a model system which realizes chemically a threshoId behavior. An analog computer was used for preliminary determination of parameter set that is attainable to steady-state, and digital computation was performed to obtain a numerical solution of the rate equation. A certain relationship among input concentration, Michaelis constant, and the substrate concentration (output) with respect to the threshold-type response of the system has been clarified, When oscillatory input was adopted to the system, the output concentration vibrated with the same angular velocity as input and it became clear that the basic behavior of the system was not altered by such an oscillatory input.