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°C, 45°C, and 48°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°C, both vasculature destruction and muscle necrosis were noted. However, at temperatures below 45.5°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°C and 47.1°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. © 1989 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.