Three metabolism trials were conducted with wethers fed purified rations to study the effect of level of readily-available carbohydrates on cellulose digestibility. In the first experiment, two trials were conducted with fifteen wethers fed five rations. The basal ration contained 77.8% cellulose, 11.1% assay protein C-1, 7.2% minerals and 3.9% corn oil. Readily-available carbohydrates supplied by a 1:1 mixture of cerelose and starch replaced cellulose in the other rations to give levels of 2, 4, 6 and 8% readily-available carbohydrates. Level of readily-available carbohydrates in the ration failed to significantly affect digestibility of ration components and nitrogen balance. Cellulose digestibility tended to be higher for the 8% ration. Energy and dry matter digestibility tended to be higher for the supplemented rations. In experiment 2B, two metabolism trials were conducted with twelve wethers to study the effect of higher levels of readily-available carbohydrates on utilization of purified rations. The basal ration was of the same composition as that described above. Three additional rations contained 32%, 40% and 48% readily-available carbohydrates (levels were substituted for equal amounts of cellulose). Cellulose digestibility was 65.3% for the basal ration. Addition of 32, 40 and 48% readily-available carbohydrates significantly reduced cellulose digestibility. Increasing the level of readily-available carbohydrates from 32 to 40 and 48% reduced digestibility of cellulose (P < .05). Digestibility of total carbohydrates (cellulose and NFE) was 62.8%, 67.2%, 65.0% and 72.7% for the 0, 32, 40 and 48% rations, respectively. The difference between the 40% and 48% rations was significant. Nitrogen retention tended to increase in the supplemented rations, but the effect was not significant. Butyric acid content of the rumen fluid was higher in the supplemented rations. In the third experiment, fifteen wethers were fed three rations to determine the effect of level of readily-available carbohydrates on cellulose digestibility following preliminary periods of different lengths. The composition of the basal ration was the same as in experiments 1B and 2B. The other two rations contained 8% and 32% readily-available carbohydrates. There was a 10-day collection period following preliminary periods of 10, 20, 30, 40 and 50 days. There were no significant differences in ration digestibility or nitrogen utilization, regardless of the duration of the preliminary period. Regression analysis of data from four experiments involving 84 individual metabolism trials indicated that for each one percentage increase in readily-available carbohydrates in the ration cellulose digestion was significantly reduced 0.5 percentage units and nitrogen retention was not significantly altered. / Ph. D.