The renewed interest in the use of hyperthermia in cancer therapy is based on radiobiological and clinical evidence indicating that there may be significant thereapeutic advantages with the use of hyperthermia alone or combined with irradiation plus heat. Authors performed the experiment using the chemically induced mammary carcinoma of rats to observe the difference in temperature changes between tumor and normal tissue during heat, and to compare the response of the tumors to radiation alone and to radiation plus hyperthermia. The results were as follows 1. Temperature of tumors was significantly higher than in the normal tissue during heating and the difference was about $1.5^{circ}C$. 2. $TCD_{50}$ in radiation alone and hyperthermia immediately following radiation was 1,282 rad and 795 rad, respectively and TER value was 1.81.