We have studied, by a nonisotopic in situ end-laveling(ISEL) technique, frequency of apoptosis in the external granular layer(EGL) of the cerebellum of immature mice by Y-rays irradiation from 60Co or diagnostic ultrasound exposure. The total number of normal cells and cells showing morphological features of apoptosis were counted. The frequency of apoptotic cells was expressed as a percentage of the total number of cells in EGL. The extent of changes following 200 cGy(1090 cGy/min) was studied at 2, 4, 6, 8, 12, or 24 hours after exposure. The maximal frequency was found 6~8 hours after exposure. The immature mice that received 18, 36, 54, 108, 198, 396, cGY of y-rays or diagnostic ultrasound(7.5MHz, 4.2mW, Ispta=7.9mW/cm2, Ispta=114.3W/cm2) for 10 or 30 minutes were examined 6 hours after irradiation. Measurements performed after y-ray irradiation showed a dose-related increase in apoptotic cells in each of the mice studied. The dose-response curves were analyzed by a linear-quadratic model;frequency of apoptotic cell in the EGL was y=(0.1349+_0.01175)D+(-0.0001522+_0.0000334)D2+0.048(r2=0.981, D-dose in cGy). In the experiment of ultrasound exposure, the frequency of apoptotic cell was 0.106+_0.130(10 minutes exposure) and 0.167+_0.220(30 minutes exposure). We estimated the relative dose of the yield from the experiment with ultrasound by substituting the yield from ultrasound exposure into the curve from the y-irradiation. The relative dose of ultrasound exposure compared with y-irradiation were 0.432 cGY(10 minutes exposure) and 0.885 cGY(30 minutes exposure). We have found that there is no evidence to indicate that diagnostic ultrasound involves a significant risk.