It is the purpose of this study to determine the degree and nature of the morphological effects that could be produced, by low megaherz ultrasound, to fetal gonadal development. As ultrasound is widely used clinically, both in diagnosis and in therapy visceral organs have the opportunity to receive substantial exposure. Although deleterious biological effects have not been reported following clinical diagnostic usage, it is also not possible to state, unequivocally, that biological effects of ultrasound cannot occur. An area in which considerable patient exposure occurs is in the examination of the gravid uterus. Such diagnostic ultrasound exposures have now involved millions of people, with nearly half of all pregnant women in the United States now being scanned for obstetric purposes, such that a significant fraction of our future population will have been exposed to ultrasound prior to birth. Although effects to the fetus have not been reported, subtle effects have not been studied. It is the purpose of the present study to inquire whether fetal gonadal development can be affected by low level ultrasound. A preliminary study showed that 1 MHz ultrasound exposures at 10 W/cm2 for 10 sec affected fetal gonads. The proposed study will involve determining thresholds at which structural changes occur to fetal murine gonads. Following identification of ultrasound thresholds at which structural changes occur to fetal murine gonads. Following identification of ultrasound thresholds at which irreversible effects occur, light and electron microscopy of fetal gonads will be investigated. The results of this study will provide a basis for assessing risk incurred by fetal gonadal structures during exposure to ultrasound at both therapeutic and diagnostic procedures.