Adult germfree rats excreted 87% more calories with the feces than comparable conventional rats, but this loss was compensated by an 18% higher intake. As a result, energy utilization of germfree and conventional rats was similar (148 and 143 kcal/kg/day, respectively), although the germfree rat metabolized only 71.9% of its dietary intake, versus the conventional rat's 80%. Germfree rats consumed 33% more water than conventional rats. Both excreted approximately 33% of water intake via feces and urine, but the germfree rat excreted 56% of this with the feces, the conventional rat only 15%. Cecectomy of the germfree rat reduced water loss via the feces, but the total daily excretion of fecal dry matter remained comparable to that of the intact germfree rat. The increase in fecal dry matter excretion demonstrated by the germfree rat can be largely accounted for in terms of water-soluble organic material. The data imply that the lower oxygen consumption of the germfree rat is not coupled to a reduction in metabolized energy, but may be an anomaly linked to cecal stasis and enlargement.