The purpose of the present study was to validate the 13C bicarbonate method (13C-M) and the doubly labeled water method (DLWM) for the estimation of the CO2 production R(CO2) in goats as a ruminant model. Indirect calorimetry was chosen as the reference method. Studies were carried out in 2 male African dwarf goats at 3 different developing stages (age: 5, 10, and 14 months, body mass: 14.6, 20.3, and 21.7 kg). Animals were fed a balanced feed 14 days before and during the studies. The isotope tracers (4 mg/kg NaH13CO3, 120 mg/kg 2H2O, and 75 mg/kg H218O; 99 AT.-%) were simultaneously given as a single pulse injection into the jugular vein. Thereafter, the animals were kept for 8 days in two respiration chambers (volume of chamber: 2.85 m3, air flow rate: 25 1/min) for the estimation of CO2 production and O2 consumption. For the determination of R(CO2) using the 13C-M samples of exhaled breath were drawn from the respiration chambers. The 13C enrichment and CO2 concentration of breath samples were measured by means of an infrared isotope analyzer. In order to determine R(CO2) by means of the DLWM, blood serum was used. The 2H and 18O enrichments were measured by an isotope ratio mass spectrometer. Urine samples were collected over 24 h to quantify renal water losses. The R(CO2) was calculated by means of the 13C-M using the area under the 13C enrichment-time curve. The determination of R(CO2) by means of the DLWM was based on the slopes of the 2H and 18O disappearance curves and the body water pool obtained from the zero time intercept of the isotope curves. The values of R(CO2) resulting from the 13C-M were found to be comparable with those from the calorimetric measurement. Smaller (not statistically significant) values of R(CO2)--92% from 13C-M and 87% from DLWM--compared to the indirect calorimetry could indicate the incorporation of 13C and 2H into metabolites other than CO2 and H2O, respectively. The body water contents calculated from the zero time intercepts of the 2H and 18O disappearance curves amounted to 66% and 63%, respectively. The body water content was found to be not related to the age of animals. The renal water loss was calculated to be 35% of the total water loss (0.76 l/d.