Experiments were conducted with the main objectives of studying: 1. the extent to which the voluntary food intakes of sheep receiving diets, differing in physical form and digestible energy concentration, change with an increase in energy demand of the sheep 2. the effect of differences in body condition and an increase in energy demand on the voluntary food intake of sheep 3. the effect of an increase in the energy demand of sheep on measurements such as the retention time of food residues, alimentary tract fill and weight of alimentary organs 4. the extent to which increases in the energy intake of sheep equate with increases in energy demand. The increase in energy demand was achieved by shearing Romney sheep, held at an ambient temperature of 13°C. A section of the work also compared the retention times of food residues using various diets stained with safranine, treated with radiocerium 144 (Ce), or potassium permanganate (Mn). There were considerable differences in mean retention times depending on the method used. Because the variation in mean retention time was lower within and between sheep for 144Ce than for the other methods, and because retention times could be determined rapidly with 144Ce, the decision was made to use it in subsequent experiments. Following shearing, there was a consistent increase in the voluntary intakes of sheep receiving chopped hay or ground hay of low digestible energy concentration. The increase in voluntary intake, with the exception of that for a hay of low protein content in one experiment, to a considerable extent met the increased energy expenditure when the sheep were shorn. An increase in the amount of dry matter in the reticulorumen and a decrease in mean retention time was observed with sheep receiving chopped hay and ground hay. Evidence was also obtained of hypertrophy of the gut, measured as an increase in weight of the empty alimentary organs, when sheep receiving chopped hay or ground hay were shorn. No evidence was obtained of cause and effect, but it appeared that increases in intake were accomplished through a range of physical changes. Evidence was also obtained that reticulorimen fill, in terms of the amount of dry matter, was unimportant in limiting the intake of chopped hay. Following shearing, the increase in the voluntary intake of sheep receiving ground hay was greater than that of sheep receiving chopped hay. The increase in the intake of sheep receiving ground hay more than met the increase in energy expenditure following shearing. The result is consistent with the postulation that the rate of removal of dry matter from the reticulorumen imposed a limitation on the voluntary intake of sheep receiving chopped hay. This observation was further supported by the greater amount of dry matter caudal to the reticulorumen, with the shorn sheep receiving ground hay, than that of the unshorn sheep. Voluntary intakes were invariably higher with sheep receiving foods of high digestible energy concentration, than with those receiving foods of low concentration, but the response in terms of changes in voluntary intake following shearing were variable. In some of the experiments, increases in the voluntary intakes of sheep receiving dried grass were small, after shearing. However in an experiment which compared the effects of body condition, and of shearing on voluntary intake, fat sheep increased their energy intake of dried grass following shearing to about the same extent as the increase in energy expenditure. In the same experiment, the greatest increase in intake following shearing occurred with the thin sheep, and it appeared that the effects of shearing, in increasing voluntary intake, were reinforced by the condition of thinness. The voluntary intakes of unshorn sheep receiving dried grass decreased as the experiments progressed. Physical restriction of the abdominal cavity by fat did net appear to be the cause of the decrease. Measurements of oxygen consumption in two experiments (values converted to heat production) were obtained before and after shearing, with Romney wethers receiving dried grass or ground hay. Heat production increased after shearing, the increase being greater for the sheep receiving dried grass than for those receiving ground hay. The evidence obtained showed that, particularly with sheep receiving hay in the intake experiments, the increase in intake following shearing would have met the increase in energy expenditure in many cases. Changes in feeding behaviour and activity of the sheep occurred following shearing. The effects of these changes on energy expenditure were discussed. It was concluded that, even where voluntary intake is predominantly limited by physical factors, these can be overriden by changes in energy demand.