The response of normal porcine fat and muscle to graduated doses of hyperthermia provided by an annularly focused acoustic source was measured. Temperatures and exposure times were varied between 43°C (20-90 min), 45 and 47°C (20-60 min), and 49°C (20 min). Response, based on histologic grading of the treated sites 30 days after exposure, was found to correlate well when mapped against several methods of estimating thermal energy deposition. The threshold for damage production was at or near 43°C. For a given temperature, a nearly exponential increase in relative tissue damage as a function of increased exposure time was found. A twofold increase in tissue damage was produced in fat relative to muscle at any given thermal dose.