The urea dilution technique for the determination of body water and body fat in vivo was compared with the tritiated water dilution technique with respect to accuracy. ease of determination, costs, and application possibilities. Nineteen sheep were used in the study, some of which had access to a roughage diet and the other to a concentrate diet. About 100-250 mg/kg urea and 0,4 mCi titriated water in 0,9% NaC1 solution was either infused or injected intravenously. Following blood sampling for urea and titrium analyses some of the sheep were slaughtered and minced for direct determination of body water and body fat. The results showed the following: I) Body water and body fat could be predicted with similar accuracy from urea space measurement at 6 to 10 minutes post-administration of urea than at equilibrium; similarly could body water and body fat be predicted with the same accuracy from tritiated water space at 15 to 20 minutes following administration than at equilibrium. 2) Body water and body fat could be predicted more accurately with the tritiated water method than with the urea method. 3) The time period necessary for the tritiated or urea molecule to equilibrate completely was independent of amount of digesta in the digestive tract. 4) Both urea and tritiated water space at equilibrium measured total water and not merely body water (water exclusive of digestive tract), 5) The urea method is not as easy as the tritiated water method due to the fact that urea has to be infused in large amounts through a catheter (injection was unsuccessful) whereas tritium can be injected fairly easily in small amounts. 6) The urea method is approximately 40% cheaper than the tritiated water method. It was concluded that the urea method is not accurate enough in critical experiments but may be applicated for body composition determinations in performance testing due to its sufficient accuracy, the costs involved, and the fact that the carcases of culled animals can be utilised immediately, which is not the case with tritiated water because of the law on radio-active materials.