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The ultrasonic threshold doses (acoustic intensity and time duration of a single pulse) to produce functional and structural irreversible effects in the mouse, rat, cat, monkey, and human central nervous systems are presented for a wide range of the pertinent parameters. There does not appear to be an important dependence upon frequency in the range from 1 to 6 MHz. Embryonic tissue is found to be appreciably more sensitive than adult tissue. From considerations of the measured outputs of commercially available ultrasonic diagnostic instruments, the experimentally determined threshold dosages necessary to produce these irreversible effects, and reports of clinical experience, it is concluded that ultrasound does not present a hazard, as currently employed for medical diagnostic purposes.