The paper describes an approach for determining nonlinearity in the pulmonary airway dynamic system, assuming a crude but linear model of the system. The linear electrical analogue representation of the dynamic system, found to be applicable only during the inspiratory phase of the respiration process, is considered as the basic model of the system. Control theory is used for nonlinearization of the dynamic process by considering the presence of an additional pressure source actuating the basic model and accounting for the system nonlinearity, which is found to be predominantly present during the expiratory phase. On analysis of the basic model, the form of the nonlinearizing signal is determined by examining the closeness between the model response and actual response of a typical patient. The use of this signal in a modified modelling of the process and in its computer simulation study is indicated. © 1982.