Some studies suggest that the prostate gland is one of the most difficult portions of the body to heat up even by radio frequency (RF) capacitive heating, which is well established as a useful means for deep regional hyperthermia because of its non-uniform power absorption. We have developed a new type of intracavitary applicator for RF capacitive heating which can be connected to an 8 MHz RF heating device (Thermotron-RF8). The applicator is a balloon-type catheter which is filled with electrolyte, such as physiological saline, 3% saline or 3% ferrous sulfate solution, and then put between a pair of regular RF capacitive heating electrodes. The heating characteristics of the applicator were examined by using an agar phantom that is electrically equivalent to muscles. When the applicator was connected to the electrode of the RF generator with a copper cord, the balloon filled with electrolyte operated as an inner electrode. The effective hot area around the balloon of the catheter was observed, and the size of the hot area was changed by the kinds of electrolytes used and their concentration. With this adaptive technique, it is possible to maintain an adequate thermal distribution at a tumor in the uterus, urinary bladder and rectum, as well as the prostate gland.