1. The utilization of the energy of the same ration by sheep and by cattle was measured using three steers and three sheep in twenty-one and twentyfour experiments, respectively. Five different amounts of the ration were given to each steer and six different amounts to each sheep.

2. No differences of any magnitude were found in the percentage losses of dietary energy in faeces, in urine or as methane if comparisons were made at comparable nutritional levels. Faecal energy losses increased and urine energy losses and methane production fell with increase in feeding level, the feeding level being defined as the ratio of food intake to that required to attain an energy retention of zero.

3. The net availability of metabolizable energy for maintenance was the same in both species at 80·4%. The net availability of energy for the production of fat was 53·5 ± 1·4 in sheep and 51·4 ± 2·6 in cattle.

4. The results are discussed in relation to the effect of the amount of food given on the utilization of its energy. It is pointed out that net energy per unit of food declines with increasing intake, due firstly to a decline in digestibility and secondly to the fact that oxidative processes operative at and below maintenance are more efficient processes than is the synthesis of fat.