The respiratory system has been shown to exhibit nonlinear mechanical properties in the frequency (f) range of normal breathing, manifested by tidal volume (Vt) dependence. Calculations of respiratory system resistance (R) and elastance (E) from pressure-flow measurements during external forcing at a given f may be ambiguous, especially if non-sinusoidal forcing waveforms are used. We evaluated the degree to which R and E depended upon: (1) analysis method (Fourier transform, multiple regression and pressure-volume loop analysis) and; (2) shape of the forcing waveform (sinusoidal, quasi-sinusoidal and step). We measured pressure and flow at the mouth of 5 healthy, awake subjects, relaxed at functional residual capacity, during forcing with the three different waveforms in the normal range of f (0.2-0.6 Hz) and Vt (250-750 ml). During sinusoidal forcing, E and R were not affected by analysis method (P greater than 0.2). With Fourier transform and multiple regression, E was not affected by waveform shape (P greater than 0.05); with loop analysis, E was slightly (less than 10%) higher during quasi-sinusoidal and step forcing than during the sine (P less than 0.05). R was least affected by waveform shape with Fourier transform. We conclude that, in the f and Vt range of normal breathing: (1) respiratory system impedance is 'quasi-linear,' i.e. despite dependencies of R and E on Vt, non-linearities are not large enough to restrict interpretation of R and E at a given f and Vt; (2) it may be possible to measure R and E using non-sinusoidal forcing waveforms available on most clinical ventilators, incurring only modest error.