The accuracy and validity of a body box system which was developed for the measurement of airway and chest wall impedance in small animals such as canines was evaluated. Input impedance was calibrated using a resistance tube and the mouth flow sensing system was designed to be as symmetrical as possible such that the minimal common mode rejection ratio (CMRR) from 10 to 40 Hz was greater than 60 dB. The validity of this system using the resistance tube and inertant models indicated that the average error ratio for the input impedance and for the flow transfer function between the mouth and chest flow was within 3 and 0.5%, respectively. The location of the box pressure port near to the oscillatory flow inlet was shown to potentially give rise to errors in the measurement of the flow transfer factor due to the direct effects of the inflow. A distance of 60 cm was adopted in the present system, which proved to be sufficient to avoid this effect. It was concluded that the body box system which is described for small animals in the present study is appropriate for obtaining separate and accurate airway and tissue impedance data.