Total respiratory input impedance (Zrs) measured by forced excitation may be computed easily from pressure and flow measurements recorded at the airway opening. The purpose of this paper was to analyse how the information provided by Zrs may be used for monitoring ventilatory mechanics during high frequency oscillatory ventilation (HFOV). We measured impedance (0.125-32 Hz) in six dogs, and in four dogs after infusion of histamine. We interpreted Zrs data in terms of a linear resistance-inertance-elastance (R-I-E) model to estimate the pressure decrease in the airways and the pressure amplitude in the alveolar region. We modelled airways non-linearities and analysed their effect at high flow oscillation amplitudes. We concluded that Zrs measurements may be useful to monitor ventilatory mechanics and to determine the optimum settings of the ventilator during HFOV.