Using ultrasound with pulse lengths of 10 (mu)s and pulse repetition frequencies of 100 Hz, the thresholds for intestinal hemorrhage in mice after a 5-min exposure range from ~1 MPa at 1 MHz to ~4 MPa at 4 MHz. With a piezoelectric lithotripter, the threshold for intestinal hemorrhage after 200 pulses was 1--3 MPa. In neither case can the effects be explained by heating. As a test for cavitation, the extent of hemorrhage in the gas-containing intestines of pregnant mice was compared to the amount of hemorrhage in the gas-free intestines of their fetuses. On the 18th day of gestation, the abdominal regions of pregnant mice (n=6) were exposed to 200 pulses from a piezoelectric lithotripter. Acoustic pulses had a peak pressure amplitude of 10 MPa and were administered at a rate of ~1 Hz. All maternal intestines showed hemorrhagic regions extending several centimeters in length. In contrast, only 1 of 43 exposed fetuses showed an intestinal hemorrhage and this one lesion was less than one millimeter in diameter. These are consistent with a cavitation-related mechanism for the production of intestinal hemorrhage by exposure to acoustic fields.