This paper describes the procedure for determination of the non-linearity present in the pulmonary arterial circulation process. A method of investigation of the non-linearity by applying control theory is first described. The basic linear model of the circulation process is then discussed and the parameters of the linear model are estimated. The basic model is non-linearized by considering the presence of an additional actuating signal to the model to account for the effective non-linearity of the circulation process. The characteristic of the non-linearizing signal is then determined by examining the resemblance between the model response and the actually observed pressure contour characteristics. The system non-linearity with respect to the determined characteristic of the non-linearizing signal is discussed. © 1976 Taylor and Francis Group, LLC.