Herbal medicinal interaction on the purgative action of Chungpesagan-tang, which is one of the traditional Korean medical prescriptions that has been most frequently used for stroke was investigated. Chungpesagan-tang had more potent purgative activity than Rhei Rhizoma alone. When each Chungpesagan-tang composing herbal medicine with Rhei Rhizoma was extracted with water, water extract of Raphani Semen with Rhei Rhizoma had the most potent purgative activity. However, Raphani Semen had not purgative activity. When Rhei Rhizoma with Raphani Semen was extracted, sennoside A content in the water extract was increased. The transformation of sennoside A of Rhei Rhizoma to rheinanthrone by human intestinal microflora was weakly induced by Raphani Semen. These results suggest that the purgative action of Chungpesagan-tang could be controlled by the addition and subtraction of Chungpesagan-tang-composing herbal medicines.