Objectives: This study was conducted to analyze scientifically the purgative action of Chungpesagan-tang, which was widely used for stroke In Oriental Medicine. Methods: The contents of sennoside A on the water-extraction of Rhei Rhizoma, the transforming activity of sennoside A to rheinanthrone by human intestinal bacteria, and the anti-costive activity of Chungpesagan-tang, Rhei Rhizoma and Yuldahanso-tang on mice were investigated. Results: Rhei Rhizoma (Rheum palmatum, from China) had dose-dependent potent purgative action. Chungpesagan-tang containing this Rhei Rhizoma had more potent purgative action than Rhei Rhizoma only. However, Korean cultivated Rhei Rhizoma (Rheum undulatum) and Chungpesagan-tang containing this Korean cultivated Rhei Rhizoma had no purgative action. Conclusions: These results suggest that Chungpesagan-tang polyprescription should be considered to enhance the purgative action of Rhei Rhizoma.