Recent reports from several investigators suggest that malignant cells are as much as twice as susceptible to heat as corresponding normal cells. Clinical trials in the last twenty years have demonstrated that superficial tumors do respond to heat, but its usefullness could not be thoroughly explored because there was no way to localize heat to volumes of tissue beneath the skin or mucous membranes. Radiofrequency current offers such an opportunity. Preliminary experiments have shown that a standard sarcoma transplanted to mice can be made to disappear without damage to the overlying skin using heat generated by radiofrequency current and these tumors have not recurred in a six month period of followup. We plan to explore this further to determine very carefully the dose response curve of normal skin, mucous membrane and many other organs and tissues. We have developed an animal tumor referral system that will allow us to test the effectiveness of radiofrequency current on many spontaneous malignancies in dogs, cats and mice and to facilitate the development of radiofrequency electrodes for internal as well as external therapy. The combination of surgical removal of the bulk of tumors, then treatment of the residual with radiofrequency heat will also be discussed.