Purified diets were offered to sheep ad libitum in two experiments. Urea was the main source of nitrogen, while energy was supplied as cellulose (20 to 72% ), starch (0 to 32% ) , or sucrose (0 to 35%) . The data indicated that (a) cellulose could serve as a suitable sole source of energy for rumen microbial synthesis, and (b) diets composed largely of cellulose, urea, and minerals could mainta...