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A variable frequency magnetic induction heating system has been developed for localized hyperthermic treatment of deep-seated tumors via power deposition into arrays of 1-2 mm diameter ferromagnetic cylindrical implants. The frequency dependence of implant heating compared to that induced directly into tissues indicates use of frequencies below 4 MHz. Above 10 MHz, tissues are heated directly from the induced eddy currents. Thermographic analyses of temperature distributions induced in tissues equivalent phantom models and in exposed animal tissues in vivo have been performed for several implant materials and array configurations. Results of the thermal dosimetry show that the majority of tissue contained within multiimplant arrays can be heated such that the temperature rises to 55-70 percent of the implant temperature differential. The resulting tissue temperature distribution is dependent on local blood flow and array characteristics, but not significantly on tissue electrical properties. Operating the system at 1.9. MHz, we demonstrate that it is possible to raise an implanted volume to therapeutic temperatures safely, in less than 10 min, with little increase in temperature outside the array boundaries.