The third lumbar region of the neonatal mouse spinal cord was irradiated with 1‐MHz ultrasound using intensities between 46 and 289 W/cm2at hydrostatic pressures of 1 and 16 atm. It was previously reported that the threshold exposure duration necessary for production of hind limb paralysis of the neonates increased with hydrostatic pressure at 289 W/cm2but not at the lower intensities, although subharmonic and harmonic signals were observed at all intensities [Frizzellet al., J. Acoust. Soc. Am.74, 1062–1065 (1983)], suggesting that acoustic cavitation is involved at 289 W/cm2Examination with electron and light microscopy reveals that permanent paralysis is always accompanied by altered spinal cord morphology and that the minimum tissue damage is altered synaptic morphology similar to that observed in the cat brain [Borrelliet al., J. Acoust. Soc. Am.69, 1514–1516 (1981)]. Tissue damage was greatest at the perimeter of the spinal cord, possibly as a result of higher temperatures resulting from heat conduction from the more highly absorbing spinal column. This suggests that hyperthermia plays a role in development of tissue damage even when another mechanism is involved. [This work was supported in part by grants from the NSF and the NIH.]