Pregnant hybrid LAF 1 /j mice were exposed to continuous-wave ultrasonic energy at a frequency of 1 MHz at a spatial peak intensity of either 2.5 W/cm 2 or 0 W/cm 2 (sham) for 20 seconds on the eighth day of gestation. Fetal weight (day 18 of gestation) and postpartum pup weight (21, 29 and 42 days post conception) were determined. No significant differences in weight were observed. This result differs from findings in others studies using outbred mouse strains. It is suggested that hybrid mice exposed to ultrasonic energy in utero may be more resistant to alterations in fetal and pup weight progression as compared with outbred strains. (Key words: experimental model; fetal weight, mouse strain specificity.)In recent years, studies have been conducted on animal systems to obtain information believed useful for estimating possible effects of ultrasound on human patients undergoing clinical diagnostic procedures. It is our view that these studies can be divided into two general categories. In the first cat• egory are those studies for which some aspect of the exposure conditions may have been chosen to approximate, in some way, an aspect of prevailing clinical conditions, but with no specific pathologic endpoint fore-identified (or specified) to be observed. The general demeanor of these studies is the lack of finding any effect attributable to the ultrasonic exposure(s), with the attendant conclusion that the clinical application of ultrasound is either with or without some risk. This argument is often considered strengthened by the fact that the duplication of clinical conditions for animal exposures in the laboratory involved either continuous-wave or long pulse regimens, rather than the extremely short pulse regimens employed in clinical diagnostic procedures. The following studies, though not exhaustive, are considered to fall into the abovedescribed category: Takeuchi et al. Smyth. 7 There have also been studies, which could be included in this category, for which effects were observed following ultrasonic exposure but which involved greatly

It has been shown that ultrasonic radiation causes morphological alteration to mouse testicular tissues, thus posing the question of male fecundity postirradiation. Three groups of 6-month-old LAF1/J mice (Jackson Labs) were studied viz. an irradiated group, (exposed to 1-MHz cw ultrasound with spatial peak intensity 10 W/cm2 for 30 s), a sham irradiated group (prepared identically to that of the irradiation group but not exposed), and a control group (animals did not leave the animal room). For the sham and irradiated groups both testes were ligated and sequentially exposed by positioning the beam axis in the center of each testis. The half-power beamwidth was 10 mm and the approximate length of the mouse testis is 7 mm. Upon recovering from the gaseous anesthesia each mouse was placed in a cage with three normal, proven females and the resulting litters were scored over a six-month period. A plot of the cumulative pups per female versus time was made for each group and a linear regression analysis was performed to obtain the best-fit straight line. The time intercept indicates the time coitus is initiated and the slope may be considered a measure of male fecundity. Though the control and sham groups had virtually the same slope, their time intercepts differed, indicating an approximate eight-day delay in initial coitus for the shams. The irradiated group showed an increased delay in coitus initiation to 11 days and a lesser slope suggesting a decrease in mating capability post-irradiation. These preliminary results suggest that ultrasound affects the males' recovery immediately following the irradiation procedure and some aspects of spermatogenesis. [Work supported in part by FDA.]