Minimum and maximum metabolism in response to cold were measured in 30 species of Australian monotremes, marsupials, eutherians and birds. In marsupials and the echidna, maximum metabolism was also determined during treadmill locomotion. These data were used to determine, for the first time, the relationships between maximum metabolism and body mass in the four endothermic groups and to compare aerobic factorial scopes (the ratio of maximum to minimum metabolism) elicited by cold and locomotion. The effect of body mass on maximum metabolism is the same in marsupials and eutherians (the therians) but is significantly less in birds. At the same body mass, there is no difference between the two therian groups for either minimum or maximum metabolism induced by either cold or locomotion. Aerobic scope during cold is significantly higher in marsupials (8.3) than in eutherians (5.1), birds (5.4) and monotremes (5.4). Aerobic scope during locomotion in all groups is almost twice that observed in cold conditions.