To analyze scientifically the fundamental formulation theory and drug 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, water extracts of herbal medicines with Rhei Rhizoma were administered into mice and the purgative action was measured. Water extracts of Raphani Semen or Cimicifugae Rhizoma with Rhei Rhizoma had more potent purgative activity than Rhei Rhizoma alone, although Raphani Semen alone displayed no purgative action. However, Platycodi Radix and Puerariae Radix inhibited the purgative action of Rhei Rhizoma. When Rhei Rhizoma with Raphani Semen or Cimicifugae Radix was extracted with water, sennoside A content in the water extract was increased. However, Puerariae Radix and Platycodi Radix decreased the content of sennoside A in the extracted water. The transforming rate of sennoside A of Rhei Rhizoma was inhibited by Puerariae Radix and Platycodi Radix. The transforming rate of sennoside A of Chungpesagan-tang to rheinan-throne by human intestinal microflora and its purgative activity were similar to those of Rhei Rhizoma. Finally we thought that the purgative action of Chungpesagan-tang could be controlled by the addition and subtraction of Chungpesagan-tang-composing herbal medicines.