Four Afrikaner , 3 Afrikaner x Friesian, 4 Bonsmara, 5 Charolais and 4 Hereford bulls within the mass range 101 – 772 kg were used to evaluate the tritium and urea dilution techniques for accurate prediction of body composition. Approximately 1,1 – 1,4 g urea/W_kg^0.75 and 30 – 40 μCi tritium/W^0.75 were infused per vena jugularis whereafter blood samples were collected at regular intervals until 7 hours post-infusion. Following sampling of blood, the animals were slaughtered. The right sides of the carcasses were physically analysed for muscle, bone and fat and the left sides plus the non-carcass components minced in toto and analysed for water, N, ether extract, inorganic material and energy of combustion. The optimum time to sample blood in order to predict empty body water was 10 ± 3,4 min post-infusion for urea and 5 ± 2,1 min for tritium, while it was 18 ± 4,6 and 234 ± 73 min respectively, for accurate prediction of total body water. The composition of the ether extract-free, empty body was 72.2 ± 2,22% water, 22,5 ± 2,05% protein and 5,40 ± 0,48% inorganic material while that of the ether extract-free dry body was 80,6 ± 1.69% protein and 19,4 ± 1,69%inorganic material. These values correspond very closely with the values of Reid et al. (1968). Linear regression equations with tritium or urea space and live mass as independent variables and the chemical components of the body were calculated. The ether extract-free components of the body – water, protein and inorganic material could be predicted with a coefficient of variation of 4 – 10% at a live mass of 400 kg, while the coefficient of variation for ether extract was 25 – 30%. The latter is clearly of such magnitude that body composition differences between bulls within or between breeds would not be able to be quantified unless measurements can be made repeatedly.