High intensity focused ultrasound (HIFU) has been used noninvasively for therapeutic applications. Previous studies have shown that HIFU when targeted on fetal rat and mice bones resulted in hemorrhage. Hemorrhage occurrence and area were found to correlate with total equivalent energy (i.e. product of energy density, number of pulses and volume of the focal region) and surface power (i.e. product of pulse average intensity, number of pulses and beam area). Continuation of the trend was observed in Opossum pups. Opossum pups (7- 8 post natal days) were exposed to 1.1 MHZ f/1 spherically focused transducer (6.3 cm focal length). Three groups of n=8 and a control group of n=8 were exposed to rarefactional pressure of 6, 4.5, 3.6 MPa with ITA values of 10.78, 5.39, 6.74 W/cm². PRF was varied by 500, 500, 1000 Hz with an exposure duration of 2 to 4 minutes. Dependence of hemorrhage on total energy and surface power suggests mechanical failure to be a possible mechanism.