Hyperthermic treatment for bladder tumor in the rabbit was investigated by the irrigation with hot saline through cystostomy after the transplantation of V2 carcinoma cells. When the bladder was irrigated with 42-44 degrees C saline, the inhibitory effect on transplanted tumor growth in the urinary bladder was significant in comparison with control groups. Antitumor effect on the survival and changes in body weight were also significant. Histological study showed the tumor cells were more susceptible to heat than normal mucosal layer. The results suggest that the hyperthermic irrigation of the urinary bladder was effective in preventing the tumor growth with minimum damage on the normal bladder mucosa. Moreover, this malignant bladder tumor of rabbit seems especially suitable for the purpose of studying the new method for transurethral hyperthermic treatment of the urinary bladder cancer.