A mechanical model was employed to examine phase and amplitude relationships between simulated pleural and mouth pressures during panting at different frequencies with and without lower airway obstruction and with rigid or flaccid extrathoracic airways. Results confirm the hypothesis that, in the presence of lower airways obstruction, compliant extrathoracic airways act as a shunt impedance. Consequently, mouth pressure changes may be out of phase with and underestimate changes in pleural pressure, affecting the measurement of thoracic gas volume.