Rates of water flux and CO2 production were measured in Antechinus stuartii in the field using doubly-labeled water. Itemized dry matter, energy, and water budgets were determined for captive animals, and were used to estimate feeding rate and energy and water fluxes in free-living animals. In winter, steady-state, field A. stuartii (mean body mass 25.7 g) ingested 807 kcal/ kg/day and metabolized 670 kcal/kg/day. They consumed about 60% of their body weight in arthropods each day. There were no significant differences between metabolic rates of nonbreeding adult (July) males and females and breeding (August) females; satisfactory data for breeding males were not obtained. Energy expenditures of A. stuartii were similar to those of several free-living, small eutherians, after accounting for differences in ambient temperature and body size. However, this comparison is complicated by the use of burrows and occurrence of torpor in some of these mammals. The energetic impact of A. stuartii in temperate evergreen forest lies between 60 and 370 kcal/ha/day. These rates are similar to those obtained for small eutherian insectivores in other communities. Water fluxes did not differ between males and females in the field, but mean flux rate in steady-state, adult A. stuartii in August (734 ml/kg/day) was higher than in July (539 ml/kg/day), apparently because they ingested rain water that fell abundantly in August. Laboratory results indicate that field animals obtain much more water than they need to maintain water balance.