Thresholds for lung hemorrhage from exposure to pulsed ultrasound have been determined in neonatal mice (24--36 h), juvenile mice (14 days), and adult mice (8--10 weeks), and neonatal (24--36 h) and young (10 days) swine. The threshold at 2 MHz is approximately 1 MPa and the threshold increases gradually with frequency in the range from 0.1 to 4 MHz. Positive and negative pressure pulses are equally damaging. Following exposure, suprathreshold lesions are not progressive with time and are repaired by usual physiological mechanisms. Although the unique sensitivity of lung tissue to ultrasound is associated with the presence of air in the alveolae, this does not necessarily implicate acoustic cavitation as the responsible mechanism. Thin sections of murine lung are more sensitive than thicker regions probably because of reduced reflection at the surface of the lung tissue.