This paper describes an apparatus for inducing local hyperthermis by passing high-frequency electrical currents through tissues between electrodes placed against the skin of the patient. The electrodes use a temperature-controlled saline solution contained by a thin rubber membrane to make contact. The resistivity of the saline solution is matched to that of body tissues. This yields a smooth transition from electrode to tissue, thereby greatly reducing the possibility of producing the skin burns which frequently appear along the edges of metallic electrodes. Use of the thin rubber membrane allows easy molding of a given set of electrodes to complex body contours for many different patients. The equipment has proven capable in clinical tests of heating bulky tumors in the head and neck and extremities without significant skin toxicity. Excessive beating of the subcutaneous fat, however, restricts the application of this heating method to tumors located in areas of the body with sparse adipose tissue. © 1981.