Knowledge of the mechanism of tissue damage by ultrasound of low‐megahertz frequency is essential for the prediction of safe dosage levels for diagnostic and theraputic use. Both ultrasonically induced heating and cavitation can cause or contribute to damage in organized tissues. Separating the effects of the two mechanisms is difficult, since temperature rise and cavitation are concurrent; also, synergism between thermal and mechanical effects has been proposed. A series of experiments was conducted under sufficiently high ambient pressures to suppress cavitation. Since all absorbed energy is dissipated as heat, elevated ambient pressure would not affect a thermal mechanism. Cat brain was irradiated with focused ultrasound at ambient pressures up to 40 atm and the acoustic emission was measured to assess the presence and magnitude of cavitation. The relation between ultrasonic dosage and the extent of damage in the cat brain under elevated ambient pressures was observed. The results are discussed in terms of the proposed mechanisms. [This work was supported in part by USPHS Grant No. NIH‐5‐P01‐HL14322.]