Exposure conditions were determined for hind limb paralysis and lung hemorrhage of neonatal mice due to pulsed exposure (10 microsecond pulse duration) to 1 MHz focused ultrasound. Spatial peak pulse average intensity and peak rarefactional pressure levels for paralysis in 50% of specimens sonicated were determined for pulse repetition frequencies of 1, 5 and 50 kHz at 10 degrees C and 2.4 s exposure duration. The results suggest that cavitation was involved in the paralysis at a pulse repetition frequency (PRF) of 50 kHz, but that cavitation took place in the coupling medium and probably not within the specimen during exposures at a PRF of 5 kHz. The results show an inverse relation between spatial peak pulse average intensity, or peak rarefactional pressure and sound on-time. Exposure conditions for lung hemorrhage were determined for a pulse duration of 10 microseconds at 10 degrees C and exposure durations of 2.4 and 180 s. The results show that the threshold exposure conditions for lung hemorrhage are much less than the conditions for cavitational or other effects reported for tissues that do not contain well defined gas bodies. In addition, the results show an inverse relation between exposure level and either exposure duration or sound on-time, suggesting that time is an important parameter associated with bubble effects.