Dietary supplementation with bisacodyl at concentrations ranging from 1 to 0.3% was found to induce both calculi and epithelial proliferative lesions, including a transitional‐cell carcinoma, in the urinary bladder of F344/DuCrj rats. In order to clarify the relationship between the bisacodyl‐associated urinary bladder calculi and the development of proliferative lesions in the urinary bladder, male and female rats were administered bisacodyl‐diets at concentrations of 0.3, 0.1, and 0.03% for 32 wk. Both sexes of animals treated with bisacodyl suffered from diarrhea throughout the experimental period. Epithelial proliferative lesions and calculus formation were observed only in the urinary bladder of male rats given the 0.3% bisacodyl diet. Proliferative lesions and increases of bromouracil deoxyriboside (BUdR) labeling indices were found only in the urinary bladder epithelium of rats with calculi, the severity of the former correlating with the calculus weight and being most marked in the dome areas, which are susceptible to physical stimulation. These findings indicate a close relationship between the development of proliferative lesions and the existence of calculi in the urinary bladder, and suggest that bisacodyl‐induced proliferative lesions are not caused directly by bisacodyl per se but are secondary to calculus formation.