The metabolism and disposition of U-14C-erythritol was examined in four groups of three male and three female, nonfasted rats each. The rats of groups A and D were germfree; the rats of groups B and C were kept under conventional conditions. The rats of group B received an erythritol-supplemented diet for 3 weeks prior to the experiment (adapted rats). The rats of groups A, C, and D were kept on an ordinary diet which was sterile for groups A and D (not adapted rats). On the day of the experiment, each rat was dosed with U-14C-erythritol by gavage (5 microCi/kg body wt; sp act 50 microCi/g erythritol). The radiochemical purity of the erythritol was 96.43% for groups A-C. Group D, which was attached to the study after evaluation of the results of groups A-C, received a more purified erythritol with a radiochemical purity of 99.46% because the data of group A pointed to a possible interference by a 14C-labeled impurity in the commercial 14C-erythritol. After dosing, respiratory CO2 and urine were collected from each rat at regular intervals for 24 hr. At termination, feces were also collected. The animals were killed and intestinal contents, organs, tissues, and the remaining carcass processed for determination of 14C-14C was excreted rapidly in the urine of all groups (range of groups A-D: 47.3-60.6% of the administered dose within the first 4 hr). Total 24-hr urinary excretion varied between 67.0% (group B) and 81.4% (group D). HPLC analysis of the urine showed that more than 96% of the eluted radiolabel represented erythritol. Conventional, adapted rats expired more 14CO2 than conventional, unadapted rats [10.9% (B) vs 6.7% (C)]. Germfree rats expired much less 14CO2 [0.8% (A) and 0.3% (D)]. In germfree rats, 14CO2 expiration started shortly after dosing, reaching half of the 24-hr excretion after about 2.5 hr. In conventional rats 14CO2 expiration started with a delay of about 2 hr reaching half the 24-hr excretion after 4-6 hr. The excretion of 14C with feces was similar in all groups (8.3% on average of all rats). Slightly more 14C was retained in the intestinal contents of germfree than conventional rats (1.9 vs 0.5%). The body retention was higher in conventional than in germfree rats (3.4 vs 2.0%). In group D, body retention was lowest (1.6%). The total recovery of 14C was similar in all groups (95.6%, average of all rats). It is concluded that ingested erythritol is efficiently absorbed mainly from the small intestine, is not metabolized to a relevant extent in the body, and is excreted unchanged in the urine. The fraction of erythritol not absorbed is fermented by the gut microflora to intermediate products which are largely absorbed and metabolized. The data support a proposed physiological energy value for erythritol of about 0.5 kcal/g.