The reliance on samples of blood or urine to estimate isotopic abundance in studies of energy metabolism using the doubly labeled water method has restricted application of the technique to animals that are either tame or easy to catch. This is generally not the case with large, free-ranging wild mammals. The use of feces as a source of body water in which to measure the concentration of isotopic markers was investigated in four female reindeer in summer and in winter. (2)H2O and H2(18)O were injected to approximately 160 parts per million excess. Samples of plasma and feces were then collected simultaneously for up to 456 h. Both isotopes were equilibrated with body water at 8 h postdose. There were no significant differences by animal between dilution spaces, rate constants, rates of CO2 production, and total energy expenditure (TEE) calculated based on samples of plasma or feces in any trial. Mean TEE was 3.557 W/kg (SD 0.907, n = 4) in summer and 1.865 W/kg (SD 0.166, n = 4) in winter.