Oxygen consumption (V O 2 ) and a number of components of the breathing pattern were measured in 14 species of non-fossorial marsupials ranging in mass from 0.008 to 30 kg. All the exponents of the allometric relationships forV O 2 , ventilation (Ve) and breathing pattern scaled as previously determined for eutherians. However, compared to eutherians, marsupials had significantly lower V O 2 and the breathing pattern was deeper (+23%) and slower (-31%). While Ve was not significantly below that reported for eutherians it matched V O 2 such thatVe /V O 2 remained mass-independent and at a level similar to that observed in the other infraclasses of mammals. Thus, it would appear that the increase in metabolic rate that occurred during the evolution of mammalian homeothermy was accompanied by parallel changes in Ve. It is suggested that these changes in Ve were mediated by a shortening of the respiratory cycle, facilitated by the eventual abolition of the end-expiratory pause (Tp), and an increase in respiratory drive (Vt/Ti). In response to 5% CO 2 all animals in this study increased their Ve by increasing both tidal volume (Vt) and frequency (f), predominantly through the removal of Tp. The increase in Ve was less than previously reported for eutherians, suggesting a reduction in chemosensitivity in marsupials. Furthermore, the similarity in slopes for Ve, Vt and f between air and 5% CO 2 suggests that the gain of the respiratory system is independent of species size within marsupials.