To analyze the structural modifications of forage grass lignin during digestion in ruminant animals, dioxane-soluble lignin fractions were isolated from the feces and rumen digesta of ruminants that received forage grasses, without any pretreatments such as ultragrinding, acid and alkali hydrolysis, and their physico-chemical characteristics were compared with those of undigested original lignin preparations of forage grasses. Both dioxane-soluble lignin fractions from the feces and rumen digesta were composed mostly of guaiacyl-syringyl lignin with p-hydroxyphenylpropane units and contained only small amounts of non-lignin constituents. When compared with the undigested original lignin preparations, the dioxanesoluble lignin fractions contained a larger amount of syringylpropane units, and a much smaller amount of bound phenolic acids and showed a lower molecular size. Dioxanesoluble lignin fractions from the in vitro rumen-digested residues of forage grass contained a smaller amount of bound phenolic acids and associated carbohydrates and showed a lower molecular size than those from the cellulose-treated residues. These results indicate that soluble lignin fragments, which are likely to be degradation products of grass lignins or lignin-carbohydrate complexes involving bound phenolic acids, are release in the rumen and then excrered in feces.