We examined the time course for measurement of basal metabolic rate (BMR; measured as O2 consumption and CO2 production) and standard evaporative water loss (EWL) for six species of small marsupial to determine the minimum time required to achieve basal/standard values. There was a highly significant effect of measurement duration on measured physiological variables with values for O2 consumption, CO2 production, and EWL decreasing with time for all species. The time required to attain values statistically indistinguishable from minimal differed significantly between species, but in general O2 consumption rate reached basal values after 4.3 h, CO2 production after 4.5 h, and evaporative water loss after 5.2 h. For 16 BMR measurements of small marsupial species in the literature, with experimental duration provided, 10 were for less than 4 h, suggesting that their BMR values might be overestimates. For EWL, three of the four published values for small marsupials may be overestimates. It is clear that appropriate experimental duration is an important component of the measurement protocol for both BMR and standardized water loss, which needs to be rigorously observed in future studies.