Data for the various components of compliance and resistance of the chest-lung system, and for inertance and natural frequency, have been collected for different species. Possible theoretical relationships have been indicated. Generally, the trends with body mass (m) are roughly as expected. A notable exception is the slow variation (∞ m-0.393) implied by the statistical analysis of the data for total respiratory (less upper airway) resistance, but here the evidence is rather meagre. The data for inertance and natural frequency are too few for statistical analysis and are included mainly to emphasize the need for further experimental work to be undertaken. Theory suggests that the dimensionless product (total respiratory resistance × lung-thorax compliance × resting respiratory frequency) should be a constant independent of body mass, but insufficient evidence has been collected to confirm this. © 1969.