Hyperthermia has been combined with conventional radiation methods to achieve enhanced tumor destruction. We combined iodine 125 seeds with ferromagnetic thermoseeds in a single plaque to simultaneously deliver radiation and heat in a rabbit model of choroidal melanoma. Initially, six ferromagnetic thermoseeds were placed in parallel on a 14-mm episcleral plaque. The plaques were placed on normal rabbit eyes and on eyes containing transvitreally implanted choroidal melanoma. The heating response was assessed in both normal and tumor-containing eyes. Rigid copper-constantan and flexible Baily thermocouples were used to monitor temperature responses. The animals were subjected to an electromagnetic field of 100 kHz, with power of 1100 to 1500 W. The thermoseeds autoregulated at 48.2 degrees C. Scleral temperatures stabilized at 45.8 degrees C +/- 0.4 degrees C (SD), while temperatures at the base of the tumor stabilized at 43.6 degrees C +/- 0.1 degrees C. Ferromagnetic thermoseeds were then combined with iodine 125 seeds. Similar temperature responses were recorded, and autoradiographic findings confirmed a uniform radiation distribution. Varying the amount or type of ferromagnetic material in the thermoseeds allows the delivery of heat at virtually any temperature. Ferromagnetic hyperthermia may provide a more simplified approach over currently available methods of heat delivery.