The purgative action of rhein anthrone in the alimentary canal was more intense and rapid in onset than that of sennoside A or of its other possible degradation products, when administered directly into the cecum in mouse. Furthermore, rhein anthrone was detected prominently in the large intestine, at the site of action of orally administered sennoside A. By pretreatment of mice with chloramphenicol, the purgative action of sennoside A was largely suppressed and concurrently the formation of rhein anthrone in the large intestine was reduced. Accordingly, it is concluded that rhein anthrone, formed mainly by intraluminal bacterial action, is the real active substance of sennoside A in mice.