Through investigation of the metabolism of sennosides by intestinal contents and feces of the rat, the main metabolic pathways were elucidated and found to be quite different from those previously proposed on the basis of the metabolites produced by individual strains isolated from the human intestine. Sennoside A was hydrolyzed in a stepwise fashion to produce sennidin A via sennidin A-8-monoglucoside by .beta.-glucosidase present in the intestinal contents and feces and sennoside B to sennidin B via sennidin B-8-monoglucosides in a similar way. The resulting metaboliites, sennidins A and B, were interconvertible under the experimental conditions used and were further reduced to give rheinanthrone, a purgative active principle of Rhei Rhizoma and senna, possibly by reductase bound to cell membranes of intestinal bacteria.