The standard metabolic rates (SMR) of species from all extant monotreme genera were determined together with body temperatures. The echidnas, Zaglossus bruijni and Tachyglossus aculeatus, of the Family Tachyglossidae, both had an SMR 25-30% of predicted eutherian levels. The platypus Ornithorhynchus anatinus, however, had a much higher SMR. The weight-independent values (watts, kg*-075) were 0.86, 0.98 and 2.21 respectively for Z. bruijni, T. aculeatus and 0. anatinus. Body temperatures were similar, however, with means in the range 31.3-32�C. The data are suggested to support the idea of a gradual evolution in metabolic capability rather than marked jumps in metabolic levels.