A computer-controlled system, based on the design of S. Brown et al. at the Ontario Cancer Institute, Canada, has been developed for uniform heating of mouse tumors. A steady hyperthermic temperature was maintained in the tumor by combining waterbath heating with RF-heating. The RF field was provided by a transmitter which was connected to two steel capacitive plates via a matching circuit. 50% isotonic saline kept at a temperature 2°C below that in the tumor was circulated in the waterbath. The saline prevented overheating of the skin and provided coupling between the RF field and the tumor. A computer program has been written to measure continually the temperature in the tumor with implanted fine thermocouples. The program also controlled the average RF power delivered to the tumor by switching the transmitter on and off at appropriate intervals. The system has been tested on tumor xenografts growing in the thigh of nude mice. A steady temperature of 42° or 44°c has been maintained in the tumor for up to an hour. The system could be used to study the effect of hyperthermia on the uptake of radiolabelled tumor-associated antibodies and the treatment of tumors by such antibodies. / Thesis / Master of Science (MS)