A heterotopic bladder attached to an Ommaya reservoir was transplanted to the back of a syngeneic rat. Following transplantation, the mucosa underwent ischemic necrosis. It was rapidly reepithelialized in 2 wk as a result of spreading of the epithelium originating from the ureterotrigonal region. By light microscopy the regenerated epithelium was indistinguishable from the normal transitional epithelium, and the cannula connecting the reservoir to the bladder remained patent throughout the observation period of 22 wk. One bladder, when examined at 22 wk, contained a transitional cell papilloma with squamous metaplasia. Although further study is needed, this model is potentially useful in studies related to bladder carcinogenesis with acetylaminofluorene.