The lower respiratory system of 10 conscious Pekin ducks breathing normally was subjected to superimposed oscillations of 1.3 to 16 Hz by a small volume piston pump. Induced sinusoidal flow (VO) and pressure (PO) signals were measured in late expiration, when the gas flow ventilated by the animal was minimum. The modulus of the respiratory impedance was calculated as the ratio PO/VO. The values obtained at the various oscillatory frequencies were compared to those predicted on the basis of a series mechanical network model consisting of resistive, inertial and compliant elements (RIC) using a least squares non linear regression method. The experimental data fitted well the frequency response of a simple RIC mathematical model. After correcting for the effects of the endotracheal tube, the mean values +/- SE of the optimized parameters were: resistance 4.8 +/- 0.4 cm H2O . L-1 . sec; inertance: 0.05 +/- 0.01 cm H2O . L-1 . sec2; compliance: 7.7 +/- 0.5 ml . cm H2O-1; natural frequency: 8.0 +/- 0.4 Hz. It is concluded that: (1) the lower respiratory system in ducks can be closely modeled by a RIC mechanical series network model; (2) the forced oscillation method can be used to investigate avian mechanics of breathing, with the birds awake and breathing spontaneously.