This paper deals with the interaction of interstitial hyperthermia (HT) and interstitial photodynamic therapy (PDT). Its main focus, however, is on a newly developed heating system; phantom studies as well as temperature-response data obtained from the in vivo experiments are presented. Heat was delivered by thin, flexible wire antennas operating at a frequency of 27 MHz. Measurements in muscle-equivalent phantom with infrared thermography were performed. Uniform heating over the inserted length of the antenna was obtained and impedance matching appears possible by simple variable air coils, thereby minimizing the reflected power to less than 20%. Light was obtained from an Argon-Dye laser system tuned to a wavelength of 625 nm at a dose rate of 75-100 mW per fiber to a total incident dose of 900 J from four linear light applicators. An experimental murine tumor (Rhabdomyosarcoma, type R-1) was transplanted in WAG/Rij rats and, after reaching an average diameter of 2 cm, the active component of haematoporphyrin derivative (HPD), Photofrin II, was injected intravenously. The tumors were subsequently implanted with four flexible catheters, through which either light or heat could be applied. Dose-response relationships for PDT alone, HT alone and PDT followed by HT were established with cure as endpoint. The animal experiments showed that with the use of low-frequency wires a good localized heat distribution in the tumors can be obtained. Moreover, this study showed that PDT and HT, in the proper sequence and only when optimal temperatures are reached, result in an augmented cytotoxicity on the tumor cells in vivo; i.e. a cure rate of 41% was obtained.