A test of the validity of the isotopic steady state relationships of the doubly labeled water (H2*O*) method has been carried out with D218O in small mammals (three chipmunks and one mouse). CO2 outputs calculated just from the rate of water intake and the ratios of the isotopic concentrations in the body water to the intake agreed satisfactorily with observed values. Moreover, reconstructed energy and material balances agreed reasonably with similar balances reconstructed for an immediately succeeding period on the same animals studied by the previously validated decay procedure. The authors conclude from an error analysis that by expressing the isotopic specific activities as abundances in excess of the body water of a subject on a given regimen, the decay procedure is economically feasible in the human with available accuracy of isotopic analyses and the present cost of H218O. The method therefore appears to be a useful tool ready for application to the field of human energy metabolism.