Sennosides, the most popular irritant laxatives, cause purgative actions in the intestine through biotransformation to rhein anthrone; however, the underlying mechanisms remain unclear. The purpose of this study was to define colonic motor actions of sennoside A with special reference to purgative action. Mice received a single oral dose of 30 mg/kg sennoside A, and the colon was removed about 6 hr later. Contractions of longitudinal and circular muscles were recorded using an isometric force transducer and a pressure transducer, respectively. In longitudinal muscle preparations, spontaneous contractions were augmented in distal colon compared to control. In circular muscle preparations, contractions were reduced in the proximal colon, but increased in the distal colon. Particularly in the proximal colon, the frequency of high-amplitude contraction was reduced. In the control group, non-adrenergic, non-cholinergic treatment decreased the amplitude of contractions in the proximal colon, but not in the distal colon. In the sennoside A group, non-adrenergic, non-cholinergic treatment only slightly depressed the amplitude of contractions in the proximal and distal colon. To confirm a causal relationship between luminal prostaglandin level and purgative action of sennoside A, the mice were treated with indomethacin. Significant changes induced by sennoside A were attenuated by indomethacin treatment. The present study indicates that spontaneous motility is inhibited by sennoside A in the proximal colon, but accelerated in the distal colon, and that effects are associated with luminal prostanoid level and only partially with cholinergic nerve mediation.