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Diagnostic levels of pulsed ultrasound were applied abdominally over the uteri of pregnant rats for five minutes daily during the first 20 days of gestation. The average energy output of the ultrasound machine at the optimum focal length of the transducer (5.4 cm) was 7.2 mW/sq cm. Postnatal auditory evoked potentials demonstrated a significant alteration in the offspring of the treated animals when compared with similarly treated, but nonexposed controls. This alteration only occurred in the high frequency range. Neither light nor electron microscopy revealed significant morphologic alterations in the cochlear elements of the exposed offspring. Although a cochlear deficit occurred in a range known to be important to the rat's hearing, several issues make this data reassuring to clinicians using diagnostic levels of pulsed ultrasound: the total amount of ultrasound exposure was higher than would be expected with human use, the defect was mild, and the absence of clear-cut morphologic abnormalities may indicate the presence of a minimal, or even reversible, defect.