This study was designed to evaluate the toxic effects of total diterpenoids extracted from the roots of Euphorbia pekinensis (TDEP) on the mouse colon and to clarify the mechanism. Dried powdered roots of E. pekinensis were extracted with chloroform, and then the extract (6.7 g) was subjected to column chromatography and preparative TLC, giving TDEP. Using the HPLC-DAD method, the purity of TDEP was determined as 85.26%. Mice were orally administered with TDEP (3.942, 19.71 and 39.42 mg/kg), after which fecal water content and colon water content were examined. Both of them increased over time after TDEP administration, accompanied by severe diarrhea. Three hours after TDEP administration, the animals were sacrificed to obtain their colons. The mRNA and protein expression levels of aquaporin 1 (AQP1), AQP3 and AQP4 in the colon were measured using real-time RT-PCR and Western blotting, respectively. TDEP significantly increased the levels of AQP3 and AQP4, but decreased that of AQP1 in dose-dependent manners. Similarly, Pekinenin C, a casbane diterpenoid, significantly increased AQP3 protein and mRNA expressions in human intestinal epithelial cells (HT-29). Histopathological examination revealed that the colon was not significantly damaged. The laxative effects of E. pekinensis were associated with the alterations of AQPs in the colon by TDEP.