Rabbit hind limb musculature implanted with nine ferromagnetic nickel-copper alloy needles was inductively heated in a 120 kHz oscillating magnetic field. Rabbits were heated every third day for a total of three 30 min heating periods. Ferromagnetic needles with Curie points of 42 degrees C, 45 degrees C, and 48 degrees C were used. Fourteen days following the third heating the animals were sacrificed and the tissues processed for microangiography and histopathology. Implanted muscle heated rapidly and achieved a stable temperature within 10 min. Two weeks after heating for three 30 min heat cycles at temperatures above 47.1 degrees C, both vasculature destruction and muscle necrosis were noted. However, at temperatures below 45.5 degrees C, histopathologic and microangiographic findings were indistinguishable from unheated, implanted controls. Myocentric granuloma formation surrounding the thermoseed tracks with vascular preservation characterized tissue at a steady-state temperature between 45.5 degrees C and 47.1 degrees C. Nickel-copper alloy thermoseeds were effective at producing reproducible, localized interstitial hyperthermia. Chronic vascular and histopathologic alterations correlated closely with previous steady-state temperatures.