This study was performed to determine whether intracaecally administered rhein anthrone and anthraquinones such as aloe-emodin, chrysophanol, emodin or rhein synergistically enhance the purgative action as has been observed for rhein anthrone and aloe-emodin anthrone. These anthraquinones were less potent than rhein anthrone. An equimolar mixture of aloe-emodin and rhein anthrone had synergistic potentiating effects because the ED50 value (50% purgative dose) of the combination was smaller than that calculated additively from the ED50 values of aloe-emodin and rhein anthrone. An equimolar mixture of other anthraquinones and rhein anthrone tended to potentiate the purgative action. These results confirmed that rhein anthrone and aloe-emodin synergistically exert a purgative action as has been observed for rhein anthrone and aloe-emodin anthrone.